Last year I built a portable shack from some plans that I found online--the type where both ends fold down into the middle. I was concerned about the tarp being too thin, but was fortunate to come across a couple of big tarps that a buddy used on his big rig. They're heavy and rubber coated, and the black rubber held heat well. I loved fishing out of it when I wasn't in my home shack.
I'm posting to see if anyone is planning to build a shack this year and needs something of this sort. I've got about a tarp and a half left (originally somewhere around 30' by 20'. They're got some holes in them, but I was able to patch mine easily. The tarps have d-rings all over them, which make for great tie-down anchors. I've also got a greenhouse cover that someone could use. It's reinforced with embedded fibers and, as far as I can tell, has no holes. I think that this one is around twenty feet long and the greenhouse was about ten feet tall. It has zippered doors on either ends.
They're free for the taking, if you can put them to good use, particularly a homemade shack.