We just bought some grey material at Wal-mart. It looks and feels like nylon material to me (similar to a stuff sack). It has a little grid pattern running through it. There was also some similar material that felt "rubberized." The wal-mart lady kept calling both of these some sort of polyester blend. The labels said "content unknown." it cost $1.50 per yard. How do y'all know when it's silnylon or some other usable material? Thanks!