Coconut fiber, about 2 inches deep. Didn't want to use that much but one brick wasn't enough so I used two. I'm sure sphagnum moss will help once I get some though. Should I mix it in with the fiber or scatter it around the surface?
Also did some tinkering and got the clamps to go over the foil panels so they're more flush, but it didn't make much of a difference by now.