While the 3D printing guys are working very hard to come up with a solution, I thought I'd try one more time for a PVC-based solution.
I saw, on Amazon, this product:
Universal Car Door Edge Guards Edge Trim 16-ft U Shape PVC 3-Color Options (Black)
Since it was PVC I thought there was some chance that it might glue solidly to a 1/8" thick PVC backing plate. So I got some.
I glued a piece of the U-channel to a scrap of PVC and let the glue set. Then I tried to remove the channel - it was stuck there really well. I got it off but some of the channel was left on the scrap PVC. So this looks good.
I cut out a backing plate very slightly larger than my test mirror, cut a piece of the U-channel, put a piece of wax paper on the back of the test mirror and slid the mirror edge (with the wax paper) into the channel.
The wax paper was to prevent any glue from sticking to the mirror.
Then I put a liberal amount of PVC glue on the edge of the backing plate.
Here is a photo of the bottom edge being glued on:
Here is a photo of the setup after the glue set with the rubber bands removed and the mirror partly slid out:
Now I need to locate a mounting ring to the backside of the backing plate. I found that if I used a combination of PVC glue and small screws the ring will stay stuck to a slab of PVC forever.
I'll show that in my next post.