I buy peel & stick floor tiles (12" usually) from Home Depot or Lowes. They come in different weights so you can choose what you want but they can be very cheap (maybe 70 cents each). One tile will make 3 bag bottoms if they are 4" x 12" (or less). They are easy to cut to size using one of your quilt rulers and a utility knife. Leave the paper backing on the tile.
I prefer to make a cover for the base...if you leave it "envelope-style" the cover can be removed for washing.
I like the stability of these bottoms -- the fabric canvas (which I bought from Joann's) was too floppy for me.