I ask for the heavy cardboard that the Quilt Shop or Fabric Stores throw out when they get to the end of the bolt. It's free for me & the store doesn't have to take it out to the trash. Some bolts have stronger cardboard than others. I'm sure that it is bendable at a certain weight, but so far none of my Bow Tucks bags sag. I've heard of people using foam board from a craft shop, too.