I have used that canvas and also a piece of foam core board cut to fit and covered with a pillowcase. And I found the bag bottom at a quilt store mentioned above that I kept for myself!! It can go in a pillowcase and I can move it to different bags.
Sewing with Nancy just had a couple of shows on bags and you can view these on her website free
www.sewingwithnancy.com and she shows some alternatives for the bottoms and for the stabilizers. Good shows.