So the problem is that the mont does not stick to the battery? Why not design a loop of plastic that fully goes around the battery. Make it in two parts with screws (1) a bottom that has the Garmin mount and then a cover that screws on and the battery. I think you have foud that glue is not the way to attach the battery to the plastic mount.

Even better but, more work is to disassemble the battery and drill and tap holes on the case. Then design the mount with matching holes and counterbores for the hex head screws.

Zip ties could work too. Make a try that holds the battery and has a Garmin mount and put some slots in the tray for zip ties.

Of these, the screws would look the best but zip ties are the easiest.