Replying to Battle ship box plans

So I'm going to build an up flat down box about 21 feet long in 3 sections each 7 feet long. Currently I have about 13 feet of lexon for the top sheet but I dont have any plywood yet for the sides. Do you think It'd work if i just used ply wood sides 1/4 inch think then just a few2x4's in the middle to hold the 2 together? or would i have to use a 1/2"? and I really dont want to pay for it but I can't figure out where I can steal it from, we have a lumber yard but it is caged in(not worried about jumping it) and i dont knwo where the ply wood is in it