I used military surplus M40 gas mask carriers, that I picked up for (if I recall correctly) six bucks each at a surplus store near Asheboro, NC. I cut the straps off and used them to re-make some new straps that I added velcro to and then sewed them to the rear of the bag, at locations where they would wrap around my crash bars. If I was going to do it again, I'd save myself some work and just use the two sided velcro tape they sell at hardware stores.
Eventually I remade them into the second version you see in the photos, for my Super Tenere. They were spray painted black with Krylon Fusion spray paint.
These bags are very heavy duty nylon, and will stand up to a lot of abuse. I made them about three years ago and I'm still using them. The paint tends to wear off over time, so I probably wouldn't paint them again, but otherwise they are very useful, simple to make, and cheap.
"No matter where you go, there you are."