The glass is just a square order of double Dimond clear float glass, A glass shop cut height and width for me. (cost $30)
laid it on the iMac and traced the corners with a very fine felt pen.
Then bought a glass cutter and cut the corners just a bit larger than they should be.
Used a belt sander with an extremely fine grit belt, and had had someone spay water at the contact points between the glass and the belt sander as I slowly wore it down to the felt pen marks.
cleaned the glass well with non ammonia glass cleaner.
Placed it back on the iMac and marked the black put areas. Used a quality frogger masking tape to cover the centre screen viewing area that was not to be painted, used a small round sticker to cover the spot for the top camera lens. Lightly just touched the areas to be painted with 240 grit sandpaper. cleaned glass again with soft no lint rag and no glass cleaner. Used ceramic black spray paint from 2 feet away, making 3 light coats, placed in a pre heated oven at 250 f, then turned it off so it would heat fast but cool slow. This bakes on the ceramic paint
Cut the glass off the screen with floss is the best way, twist a few stands together, but it will take a lot, do not use heat at all.
Do not use UV fluid, it is a horrific toxic substance that your bring into your home.
I am using just clear non acid based high quality silicone (fish tank silicone) to glue the glass back to the screen. Just a thin even coat that I dropped 1 corner on first and slow laid the rest down. moved the glass in different directions to work out the extra chalking. Final positioning, wiped off the extra oozed out chalking with vinegar then let it dry over night. Then reinstalled the screen, Note that it takes 2 weeks for the silicone smell to go away and would not leave it in my house for the first few days anyways, a garage is best, cold will not affect the bond or the cure. I want to try wild colours instead of black, but it has to be ceramic paint, for all other types will loose bond with the glass after a short while, really is a low cost fix.