Which wood to use for sides and roof?

I want to build a 5' x 8' pop top trailer like George Teagues built and wanted to use marine grade plywood. I called Middletown lumber and the only thing they could find was 5' x 10' sheets at $190.00 each. They told me that if I did not use marine grade plywood for the trailer it would eventually rot away due to the voids in regular plywood. My question is, do you have to use marine grade and if mot, what can you use, I would like the sides and roof to be 1/2' or I could frame the sides and use a 1/3" ply for covering. Any suggestions would be appreciated, I realize that to make this 5' wide and 5' high I will have to glue solid 4x8 ply to the size needed, thanks Gary from Harrisburg, Pa. 
