I couldn't find plastic that I felt was stiff enough for bag bottoms and yet slightly bendable until I found some plastic binders at Hobby Lobby. The plastic was just right-I just marked with chalk and then cut out pieces of the binders in the sizes I wanted for my bag bottoms. They fit just right, come out easily for cleaning, and best of all--cheap! I got 4 bag bottoms out of one 2-dollar binder!