Bill, a few years ago when I tried moving my driver’s mirror I found one set screw frozen and couldn’t budge the mirror. I ended up in a long effort to drill out, rethread, and replace the set screw, simply so I could get in and out of our home pad without having to hold the air up button so that mirror would clear and go over our 6’ side fence in the process; moving the mirror inboard was a more palatable solution.
Like you, I discovered that those screw port covers are hard to come by, and are painted plastic that’s not well compatible with screwdrivers. I tried cutting off stainless screw heads to plug the holes, but the fit wasn’t quite right. One day at our local Ace Hardware, I perused their specialty fasteners on an end of their bulk trays aisles. I found nice little chrome plugs that turned out to work great. They have prongs on the back that can be tweaked as necessary to keep the plug in the hole, but can be carefully pried out with a knife if needed. My only gripe is that after many years out in the weather they’ve begun to rust, so I wish they were stainless; the mirrors’ chrome breaks down too… anti-UV polish staves that off. But the plugs are not expensive and easily replaced.
Try checking the specialty trays at an Ace or other hardware source.
Joel