It depends how fussy you are or wether you want to be hi tech/extreme light weight or whatever. Lorry tarp (the sort they use on curtain sider's) is very very tough I expect similar to what Ed's hauling sack's are made of. Scrounge some from a haulage company they ofetn have bits lying about, you can heat weld it together or stitch it and seal it, I am sure you couldmake up a simple sort of bag/haver sack type scenario. When I was in the scouts we made bivvy bag's and tarp's from thick polyhene scrounged from the carpet shop (we werent into gore tex in 1973
), we welded them up with a domestic iron, with newspaper over the joins so it wouldnt stick to the iron (crude form of heat sealing machine they use in food factroies etc)