I am going to look at this 24" x 36" x8" mixing tub at TSC. I am thinking for the inlet side I will just leave the concrete lid in place, sit the tray over it, backfill around it then fill the tray with mulch. My idea is that it will be easier to rake enough mulch out to lift the tray than to re-dig the clay soil. Of course in a number of years the mulch will no longer be loose and will have small hostas growing in it but still easier than digging clay. Hostas have taken over that shady part of my lawn. I keep them mowed and they stay small making a nice ground cover.