I'm not familiar with the mondo bag, but I have made several bags and I really like to use headliner in them. Headliner fabric is what auto manufacturers use on the inside of the roof of a car. It gives the fabric
some stiffness and stand-up-ability (yes I did just make that word up
) while staying soft and foldable at the same time. It's easy to sew and quilt through.
You can get headliner fabric at Hancock's, Joann's, and I'm sure other places as well.