To sum it up, I rolled the contents of my FRAKTA bag onto the Parisian streets. Got a big (nasty) hole in it. All because it shifted! I was glad it was not my underwear laundry day…
So basically, get any piece of wood that is above 12mm thick. I had leftover 1/4 round pine.
Cut a piece of 35cm.
Make a notch 6-8mm deep, 15mm wide starting at 20mm on each side. You should have 28,5cm in-between.
Then, just place the piece of wood in the straps, adjust them a bit, and get rolling!
The bar will hold the bag in place. You can glue it or sew it for more safety. I find it good enough this way, so I can remove it anytime.