I decided not to attach the top. It is 3/4 plywood with holes drilled randomly in the top to let the moisture out. Then I covered the board with some foam I had ( about 1/2" thick) and 2 layers of mattress pad. I decided the weight held it down and this way if an area was stained I could rotate the board. Also thought it would be easier to replace the two layers of old sheet I stretched and stapled over the top
When I thought I was going to attach it I was going to get the small L bars from Home Depot and attach them on each side. Good luck, let me know what you end up doing.