In my nappy bag post I mentioned having to buy new interfacing. This was fusible fleece, which is commonly used in bag-making as well as other items where you want quite a padded finish – like slippers, for example. It looks like the wadding you’d buy for quilting, but the ‘fusible’ part refers to the fact that one side is coated with an adhesive that sticks to the fabric on ironing, just like regular interfacing.
Fusible fleece is extremely useful for making a bag sturdier; however it is much harder to get hold of than regular interfacing. I have sometimes found it in Decorative Cloth in Leamington, but otherwise you will need to try the internet (for example, u-handbag). This time I could only get hold of fleece that was fusible on both sides which presented a bit of a problem as I often needed to finish one piece before adding on the backing. I ended up not fusing it (in case the other side fused irrevocably to the ironing board!) and instead sewed it to the bag as I added the other pieces (in this project, for example, the side handles). Alternatively you could sew it on round the edge of the fabric to hold it in place.
This led me to wonder whether in a pinch, you couldn’t actually use quilt wadding in the same way. I plan to try it next time I’m running short of the fusible sort! Another alternative if you need structure rather than padding, is to use pelmet stiffener or buckram which seems to be easier to get hold of. This is very stiff (and thus harder to manipulate, for example if you need to turn the whole bag right side out through a small hole) but can be sewn in. I’ve only seen it in quite narrow pieces but it’s well suited to smaller projects.