Ace is the place I have my bag bottom cut to side. Approx price per bottom is $2.35 - $2.85. They figure by square inch I believe. I just tell them ie. 3-1/2 x 14. Or whatever size I want. I then make a sleeve for the plastic and Velcro it to the bottom of the bag. Easy to remove if you wash the bag.
Judy in Phx, AZ