To heck with the tape. I'd use a little JB-Weld on the aluminum magwell flare. First, I coat the pistol frame/magwell with some a lubricant....I like pam cooking spray or vaseline....just a thin coat, wipe off any excess. Epoxy won't stick to an oily surface, or at least that's been my experience. Use masking tape to tape off the magwell flare, mix up some JB-Weld, slather on a little around the mating surface and install the flare. Carefully wipe off any excess that may be oozing out....let it start to set up until its like modeling clay or a little harder, then use an exacto knife to carefully do any additional cleanup, then let er' finish curing.
Should be a 'form fit' now and still removable.
If you don't want to do it, send it to me....I'll do it.
Or, you can do it the easy way and use the tape method.....nothing wrong with that.