I make a pillow case pocket looking thing the size of the bottom of the bag and slide the firm material in the pocket case. You don’t know it is there if you use the same fabric as the bag. When I launder them, I just pull the pocket out and remove the cardboard, plastic or whatever I have used and wash the bag and the pocket. I do this for purses too and use the lining fabric so it is not noticeable. Some times I put a bank envelope with cash under there for emergencies, uhh like quilt shows �� or when I am carrying larger amounts of money.