For a black 1/2" sheet of HDPE,
McMaster wants $216 http://www.mcmaster.com/#plastics/=85xn8y
Trident Plastics in Bucks Co, Pa quoted me on the phone at $150 for black and $140 for natural (white)
http://tridentplastics.thomasnet.com/ca ... 100%7C1001
Trident is in bucks county Pa. Not too far for you Jersey guys. I can't imagine how much it would cost to ship a 4x8' sheet. I found a place in Cedar Grove Nj called Emco Plastics that sell UHMW. They may carry HDPE too. http://www.emcoplastics.com/index.cfm?f ... gory_ID=47
Like Roadie said, There will have to be some cross bracing fab work to keep the plastic from warping or flexing too far. Gonna' be a little trial and error but I think I can make this work.