I used luan(3/16" underlayment) and 2x2's, worked fine, but should have had 2x4 angled braces come out to under the outer 2x2's.What did you make your poop board out of? I’m considering using that white “shower board” supported by a framework of 1x2s underneath. It seems like you’d need at least ½” of plywood for it to be self-supporting and that could get really heavy.