Go to your local sign shop and ask if they have a scrap/misprint bin. Depending on who's at the counter I sometimes get stuff for free, but even if I have to pay something, it's a small amount. The sign material can be used for bottoms. I have used the thin stuff in the sides, but you can't sew thru it so it took some planning. You don't want to use cardstock because it will just crease and fail.