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You don't mention whether or not your character needs near-term access to the item, I therefore give you...

Roman Concrete

The formula for which we still haven't discovered and don't fully understand — especially since modern concrete doesn't last anywhere near as long as Roman concrete has.

Build a form, pour the concrete over the item, fill it up like you're trying to hide Jimmy Hoffa. Nobody gets in or out. Not a bird in the sky, not a rat on the ground, nary a worm or a fly or even bacteria not previously present on the item.

If you don't want the concrete touching it, pour it like a dome over the top.

Boom.

Oh, rats. I just noticed you want this to be a minimal-person job. My answer requires a small team of people. Sorry.

Edit: After the addition of the item size in the question, the problem is now one that a single individual could solve in a half day or so, so long as the technical expertise and raw materials were both available to the individual.

Romans were already able to build walls and roofs. Those will cover the sides and the upper part of the thing. No need for windows, of course.

For the ground, spread generous dosages of natron, already available to Egyptians.

Historical natron was harvested directly as a salt mixture from dry lake beds in ancient Egypt, and has been used for thousands of years as a cleaning product for both the home and body. Blended with oil, it was an early form of soap. It softens water while removing oil and grease. Undiluted, natron was a cleanser for the teeth and an early mouthwash. The mineral was mixed into early antiseptics for wounds and minor cuts. Natron can be used to dry and preserve fish and meat. It was also an ancient household insecticide, and was used for making leather as well as a bleach for clothing.

The mineral was used during mummification ceremonies in ancient Egypt because it absorbs water and behaves as a drying agent. Moreover, when exposed to moisture, the carbonate in natron increases pH (raises alkalinity), which creates a hostile environment for bacteria.

Stump.

1. Find dead tree stump of appropriate size. 2.5 m would work. A burned stump is ideal; if not, burn the outside.




2. Cut it off at the ground or pull it up by the roots if you can.




3. Hollow it out.




4. Lower hollow stump over pole.




5. Pile dirt up against base of stump / roots. Move vines there. Fertilize new vines with whatever you might have handy.

Now your untouchable pole has a sheath. It will last as long as a burned stump lasts. Charred exterior will dissuade insects which might burrow in and eventually traverse the stump to the hollow interior where your pole is.

The benefit to this method: the pole is also camouflaged as a worthless and unremarkable item. It will attract no attention. If I were traipsing thru the woods and I found a bronze box that @Kilisi built, standing in the middle of nowhere, I would think "Treasure!" and return with my buddies and their hacksaws. Or just take a nice metal box to keep my stuff in. Not so the rotten and burned stump - not even good for firewood.

The item is impervious to fire

Well there's your answer. Build a fire pit around/on your item, lined with the same materials you'd use for a forge. Light a fire, and then every 8 hours or so, return and top off the fire with thick pieces of hard-wood in a compact load to keep the fire going.

It'll be expensive to maintain a permanent fire, but I guarantee that nothing will want to touch an item in the center of a fire.

First Line of Defense: Chemical Warfare

Chemical animal repellents are effective over very large distances, deterring animals before they can even see the item. Your strategy should start there.

And ancient romans had tons of options. Most were derived from plants and animals, and most are still effectively used to this day. They usually work either by overwhelming the senses or activating a danger instinct. I imagine your ancient roman concocting an absolutely putrid, foul brew and just dumping that eye-watering mess all over the place. Here are a few possible ingredients:

• 
  Peppermint and garlic were cheap and commonplace spices back then, but are unbearably strong for many pests.


• Some other common pungent repellents included castor oil, vinegar, and rotten eggs.


• Tropical products like pepper and cinnamon would've been more expensive imports.


• 
  Predator urine instinctively stops many animals in their tracks. Depending on the province in the empire, you could probably purchase some excellent bear or lion urine near the local amphitheatre.


• But some human urine would do nicely, and was cheaper and easier to get. You might buy that at the laundromat, but it would be fresher to just to scoop some out of the collection vats down the street!

But if your character is industrious and obsessive enough...

Why not do all the things?!

If critters somehow got past that awfulness, then they'd face the good stuff the others mentioned:

• Dead zone! Sure, can't hurt to scorch the area and salt the earth.


• Ring of fire! But with a change -- a trough or small moat of burning oil, with a nearby vat to provide a steady supply, would be far more reliable than a wood fire. Like a big circular oil lamp!


• Surround it with intimidating scarecrows. Animals are kinda dumb. Psychological warfare!


• Put bird control spikes everywhere a bird could land.


• Seal it up! Paint the thing itself with a nice thick coat of tar or wax. That by itself should be enough to stop small animals such as insects and worms. (Warning: keep tar/wax away from ring of fire!)


• Finally, lower a concrete and/or metal enclosure over it. That easily solves a lot of problems, including your insidious gopher threat. As a side note, a Roman metal box like this would probably have been made of lead.

Nothing would deter all animals from entering an area. Your only option is to have an area that can't be entered. Metal boxes are well within Roman tech.

Safes have been used for a long time to stop anything from touching valuables, we still use them today.

You could dig down a foot or two in an area and pour concrete in that space to make a concrete floor. and maybe put a sheet of metal down below or on top of the concrete. Then put soil on top of the concrete and then rest your pole on top of the ground you have put on top of the concrete.

Then you will have prevented creatures from coming upwards from the ground below the concrete.

All you will have to worry about is critters coming along the ground from the sides or flying through the air.

you could use poles to erect netting or a fabric tent around and above the object, so flying and walking critters couldn't get through the net or tent to reach the object, unless they were strong enough to break through or tiny enough to slip through tiny gaps in the netting or cloth weave.

But you, a human, could always untie the tent flap to get in or look through the netting to see that the object is alright.

To keep big critters away you can make a rock garden around the concrete area, selecting hard and shape edged rocks, with the sharp edges pointing up and the spaces between too small for a wolf's or wild cat's paw, and the width of the rock garden too wide for a lion to leap over.

Most of the local humans will go barefoot and would cut their feet on the sharp rocks. But you can put on sandals with thick wooden soles when you want to get to the object, or use stilts whose bottoms are thin enough to fit between the sharp rocks.

And you can use foul smelling chemicals to keep lifeforms away as others have suggested.

Or maybe surround the area with a ditch you keep full of fire as others have suggested.

And you may want to have duplicate poles inside your protection area, or maybe duplicate protection areas with duplicate poles.

Something is bound to happen sooner or later to destroy your defenses. A forest fire, earthquake, storm, etc., may destroy the tent or netting, or maybe a raiding party of pillaging barbarians will see your spot and figure out how to get in and get what is guarded.

So the shorter you have to keep the pole guarded, the more likely you are to guard it successfully.