GPW, yeah I agree, and I think we are just looking at it from two different angles: Simply plywood floor is easiest, quickest and probably thriftiest, or if MaryC chooses to do a built up insulated floor, then a top ply is a good idea. Different page, same book.
Dirke, T-shirts sound like a thrifty alternative, but perhaps more labor intensive. I was able to buy an arm full of used king size bed sheets at a yard sale for dirt cheap and have been using them as dust covers for my building materials in the meantime. While I will probably use duck canvas (or epoxy/glass) for my outer skin, I may use the bed sheets for things like lining the inside of my tongue box, or making some lantern travel cases.
I have access to lots of cast off pallets and considered using them for building materials, but the problem is they are rough. I mean, if they are not busted and full of nail holes, they are cracked, various species (sometimes within the same pallet) and mostly short lengths... and they are rough. I tried to get some oak stringers from an oversized pallet back at the beginning of my build, and just busting out the slats and pulling the nails about killed me on a hot day. Once I had them in my hands I asked myself why I had bothered; wasn't worth the effort for twisted, split, rough, nail hole riddled wood, and the middle stringer wasn't even oak. Mind those nails, too. Half of them will break off instead of pulling, and now you have your saw blades and planer knives to worry about. There are easier sources of scrap or surplus thrift wood.
Cut some pallets up and bring them camping as fire wood is about all they are good for in my opinion, unless of course you have access to premium grade pallets. $.02