you'd probably have to put something under the top sheet, whatever it may be, so that it's high enough to be flush with the sides of the ladder. if you were to do that, you could probably drill holes in the rails of the ladder, then countersink bolts through the top sheet, through whatever you use to raise it up, then through the rails, to secure the topsheet to the ladder. or secure it through the sides of the ladder into the topsheet and whatever. and unless you're planning on building a plaform for it, or leaning it up against a retaining wall, stairs, etc., it's going to be a flat box a few inches off the ground, which would probably get boring quick. if you go forward with this, keep us updated. it sounds like a good idea if you put enough time into it.