I have used both bosal or soft and stable to make bags. I like using this method for side seams and avoiding bulk.
https://www.youtube.com/watch?v=fwdbjufqG3E
You can cut the strips in smaller widths so it reduces bulk. Make a sample strip first to see what size to use. You may find that one layer of batting in the strip works better than the double folded batting. Simply adjust the width when you cut the batting.
The other tip is to make the strips the width you want. Then quilt down the strips with a 1/2 inch spacing. The quilting will add stability to the bag.
Good luck & have fun!