When I make patterns for sewing projects, I use a bed sheet. Last one was from Big Lots for about $2.00. Make sure it isn't a fitted one
I usually pick a sheet pattern I hate, that way I won't accidently use the material for anything else, & if it's teal all the better (I HATE teal, and it usually contrasts with ANYTHING I may sew).
FYI: I usually touch up the edges with frey check or similar product.