go to the hardware store and get a piece of "PLOOD" plasic wood composite....then make a box frame and put that on top...counter sink the screws and your done. Plood is what all kinds of huge ski resorts use 4 their boxes. Not too expensive either.
I think is like 40 bucks for a really long piece like 16 feet or something...it all depends. BUt if you get it use the opposite side you would usually use like on a deck or something....Just ask the hardware store guy which side to use and use the opposite...Its so sick and lasts so long. (Plood is a plastic and wood composite that people use for decking...its better than actual wood for decks cause it lasts so damn long. But if you use the opposite side for a box its so sick.)