Here's just an idea. Visit a metal recycler or several. What you're looking for are the ends cut off of big propane tanks. I have found sizes from two feet diameter to five feet. The larger the tank end, the shallower. They cut them off so that they can split the main part of the tank and break it down. The big ends are up to 1/2" in thick. They are easy to drill, so it's not hard to make drain holes in the bottom. (The make outstanding planters, too.)
They can be partly buried, set on the ground, or set on a short section of big iron pipe. Or have legs welded on.
Probably the cheapest ground treatment is irregular gravel or crushed stone. Or 12" concrete pavers. If you set the pavers on sand, you can come in at the end and pour dry concrete ready mix over it and between the pavers, keep it wet for a day, and it will set up and keep the gaps filled.