double plate the bottom, then due all your layout on 12" centers, im assuming that your not planning on making it too tal so 12" centers should be fine. all fasteners should be stainless if its in your budget, Depending on the outside dimensions, you might want to consider using 2x6's to mininmize the barcing required on the top, once framedskin the inside wiht 3/4 plywood, you could also skin the outside to increase strength