Since I cannot turn my neck around very far, I have always had over-sized clip-on rearview mirrors to see behind me while driving. The problem has been they add a lot of weight to the existing mirror and when it gets warm in the car, the mirror stem goes limp.
Last week I ordered some custom cut polycarbonate mirrors to the size I need (21 x 2.5 inches,) along with a really stiff mirror stem, and some fast curing acrylic casting resin, to create a completely custom light weight mount and mirror.
Casting the mount was pretty easy. I just made mold using modeling clay, mixed and poured in the resin and 10 minutes later it was hard enough to break out of the mold.
Since I was worried that acrylic glue might damage the mirrored surface, I instead attached the mirror to the mount using 3M high temperature tape. To test that theory, I left it in the hot sun, windows closed and when I checked it 5 hours later, there was no warping and no drooping. Summer will be the real test. I think this project was a success. Total cost was around $75 with enough leftovers to make another one.