Keep in mind, adding something to push the top of the holster away from your body will result in the gun being directed further towards your body during reholstering.
The most often suggested method of reholstering an AIWB carried gun involves rotating the gun and holster slightly, so the muzzle is not pointed at your body while you push the gun into the holster. Modifying your holster in the fashion you are proposing will make this potentially life saving maneuver much harder.