so I very much enjoy 'McGivering', do it all the time to my motos and backpacks...
so, you could put some sort of 'stop' on each rail, to prevent the bag/strap from sliding down the rail.
could be some plastic collar OR simplest, easiest and would likely last a long time - Duct Tape.
so, I did a quick example on a saddle I had laying around - several wraps of Duct tape on each rail, at a position which holds the bag strap above the 'stop'
my example is only a few wraps, my estimate a few more would give enough 'height' to prevent the strap from sliding over the stop.
I would also use black duct tape - i used silver and magic-markered the tape for visibility...
Saddle rail stop - using Duct Tape
I expect the width would only need to be 1/4 - 3/8 wide, and the 'height' maybe 1/8 ?
easiest done with saddle off, but patience would work without removing the saddle ... easy enough to mark the rail, when the bag is in proper position...
ymmv...
Ride On
Yuri