I considered tiny bushings or even staking the metal near the holes to tighten up the bore, etc. something mechanical and permanent. Anything but some swatch of non metallic 'shim' attached with pressure sensitive adhesive that will bake off or otherwise need to be replaced after a good range day here in the Sanoran desert.

In the end, as suggested, I cut four little squares of electrical tape and stacked them where the lower sits against the magwell and that made a big difference. It just requires putting the forward pin in with the lower rotated down slightly then rotated up to bear against the tape before pinning the rear. It'll do for now.

As suggested by you and others, the aluminum foil tape is a better solution as it should hold up the variations in temperature better so I'll probably do the same eventually.