Probably depends on how many you're packing in the space I suppose and maybe what's around it. We built two bunks almost 20 years ago with some NRCS cost share and haven't had to touch them. Took the topsoil off with the dozer and then laid down woven geotex, covered it with some 2 inch rock back to level and then topped with ag lime basically and packed like concrete (local place calls it man sand.) Think pitchers mound, but not quite that pronounced of a hill. Center 4x4 posts with an 8 foot monoslope roof, the 4x4's allow the roof structure to give a little in the wind and not stress it as much. Can make it as long as you want with posts after each 8ft bunk. Pad is just about right to where they're crapping off the back of the pad so pad doesn't accumulate.