how can I seal the center of a tractor tire?

hello all,

I have a tractor tire I would like to re-purpose into a water feature for some diversity in the backyard. Im wondering how to seal the center of this so that it will hold water. My backyard gets very hot in the summer, so Im wondering what would work with the expansion and contraction of the tire as it heats and cools. Open to suggestions here - Ive thought about packing in clay; maybe bentonite; a concrete plug or maybe a piece of pondliner sealed in with silicone caulk.

I would try pond liner with silicone caulk and a dozen wide-head screws around the inner lip. Also remember that there will be a lot of weight on this, so place it with some good earth bottom support.

I would just line the whole thing, up to the rim, rather than trying to make a joint under water. It won't take that much more pond liner, and you can use cheaper builder's plastic instead if you put some carpet or fabric scrap between the dirt and plastic in the middle, and then an interior liner between plastic and water, perhaps of carpet scraps. This technique is called carpet sandwich (plastic sandwiched between two layers of carpets) and can be used to line any small pond.