plywood... and then a wood frame made of 2x4s, i dont understand why there are so many questions about the construction of boxes and rails, in most cases all you have to use is a little common sense.
dude dont make it 4 pipes wide, build your box, put two pvc pipes on the side, and than buy some white borad and cut it up and put it in between. its amazing
just do what thejackall1177 did on his new rail...just made a rail that was a little wider which makes it more fun...what you do is use a 2x6 and use like 1.5 inch pipes and 3 of them or sumtin and its nice and wide, skinny box type, but not HUGe
