Mostly, I use the small mesh plastic sheets from JoAnn's (or wherever I find them cheap)... measure the size of the bag bottom, cut 2 from the sheet and make sleeve with fabric that matches the lining of the bag and slide them in. Then I hand stitch closed the small open end. In the past I also cut down a large rotary cutting mat, when it got warped, and used it instead of the plastic canvas... Also, once in awhile my DH will buy plexiglass (for use in very large picture frames to keep weight down) and he will take the leftover plexi and cut it down to size and sand edges for me to use in bags, too.