Well the one I usually use is like 6 feet of foam, and it's all held up by a mesh net. Under the net it's just hollow empty space for at least a few feet. I guess that's too advanced too make but it's a lot safer then just a box with 6 feet of foam, because I've hit the bottom before easily. Climbing out isn't really a big deal, I'd rather have a tough time getting out then breaking my ankle on the bottom, right?