Let's assume they're 2" thick and 18" wide. (If they're more or less, you do the math.) Dig a circle that's 22" in diameter and 8" deep. Lay in either some Filter Fabric or landscape fabric (or even some 6 mil thick plastic, but poke a few pencil holes in it.) Then, in 2" layers, lay in some 3/4" crushed stone until it's 6" deep. Tamp every 2" with a tamper or a 4" x 4" piece of wood. (THe kind that's used for a dry well. Any gravel yard or quarry will carry it.) After the 6" has been layed in, lay a sheet of fabric (or plastic with some holes in it) across the stone. Then, lay in your discs. Then lay in some nice, landscape gravel around the disc for the remaining 2". When you're done, 'cut' any extra plastic or fabric that's 'sticking up' around your hole. Ta-da! You will slow down the rotting process and your discs won't be sitting in water or in contact w/the ground at all. By having the plastic or fabric at the 2" level, you can remove the discs for sealing whenever you want, and by just lifting up the sheet, you can easily remove the landscape gravel only for placement back down later.
The fabric or plastic (with a few holes in it) at the bottom will keep the surrounding dirt from 'polluting' your gravel and prevent weeds and grass from growing. IF you get grass or weeds, you can spray Weed & Grass Killer on the stone, or pull them up.
-::(^^^^^^^^)::- landscape gravel surrounding your disc ---------------- fabric (or plastic w/holes) at the 2" depth -::::::::::::::- 3/4" gravel -::::::::::::::- -::::::::::::::- --- ---- --- ---
^ ^ fabric (or plastic w/holes) at the 8" depth, also wrapping up the SIDES, to the top of the hole