When developing the Francli Day Pack we experimented with various waddings to go inside of the straps and back panel. Ali from Francli Craftwear uses a very thick hardy foam, but this wouldn’t be suitable for sewers using domestic machines.
It is also a personal preference how bulky you want these areas of the bag to be, we definitely advise doing some sample bits to test the thicknesses with your machine.
The wadding we chose to use is this one. It is made in the UK from wool and Ingeo. It is quite a lightweight wadding so we layered up our pieces to create one thicker piece. If you purchase 0.45M of wadding like the pattern states you can get 5 layers of each pattern piece. We found this was a good thickness for our machines here and the fabrics we were using for the bag. The great thing about this wadding is when you layer it together it melds into one piece very easily.
Other waddings you could use would be a traditional quilters wadding, like 100% cotton or bamboo, these are readily available in lots of craft shops, but again we would recommend doing a test piece to see how many layers you want to go for.