Now, I have no idea what the material is called, but on a couple of bags I have purchased there is a rubber texture coated fabric that is used on the velcro straps that secure the frame bag to the frame. The rubber coating means it grips the frame and does not slip.
I have searched a few outdoor fabric shops, but cannot find it. The image below hopefully shows the stuff I am talking about.
If you zoom in you can see the textured coated fabric on the velcro straps.
Here is another pic. Again, if you zoom in you will see it on the top of the bag where the bag meets the seat post
http://bikepack.pl/en/seatbags/9-repack-x2-medium.htmlIf anyone can find me a supplier for this stuff, or tell me the actual name of it, so I know what to google, please do reply.
Thanks