from my experience on airsoft fields i can tell you, the best way to make an airsoft "fort" is to take ply wood (ply wood, not composite board, composite melts in rain. plywood will split if it's cheap stuff, not that it matters much once it's nailed in place.) and nail those boards on to some wooden crossbeams that you've dug into the ground. if you want a roof i'd suggest some kind of plastic board or plexi glass over wooden supports, but if you want to be particularly cheap you can replace the glass or plastic with a plastic tarp nailed to plywood. what you need to remember is you need to use nails while doing this, you also need to make sure that you dig small holes in tight soil to put the support beams and struts in, if not the whole building could come down, possibly hurting someone. also, if you have any cement you'll want to use it to fill the holes you put the support beams in, and possibly the floor if you're feeling up to it.
