Which one (backstrap) did you settle on? Remove it, set it aside.
Take one of the ones you won't be using. See the four wider spots at the top/bottom on each side? Use a heated screw driver blade to make a small flattened area towards the trailing end of each rail. Not the end that first slides into the grooves in the frame. You want the leading edges to be in good/original condition so it inserts smoothly as you slide it into position. The very slightly flattened lower corners of each of the four wider edges should lock it into the frame rails tighter.
Try it on one of the unused inserts to see if it works and you want to try it on the insert you use in the pistol frame. Let me say again, it shouldn't take much deformation of those small rails and just a small section of each rail. Go to far and getting the insert into place may be impossible. Good luck.
Did something similar years ago to the plastic pins in my Ruger 10/22. They were loose enough (the two that hold the trigger group in the receiver) that they would start to work out as you fired the rifle. No issues since.
People do it on loose fitting metal parts, too. Not with a hot screw driver blade, but with a hammer and punch. I had to make little dimple marks in the rear sight dovetail on my wife's first CZ75 Compact. The rear sight was extremely difficult to get out and then the replacement sight was a very sloppy fit. I used a punch/hammer to dimple up the metal in the bottom of the dovetail and then got a good tight fit when pushing the new rear sight into place.
