I have found myself in a bit of a timely search for anti-reflective coated glass material. Flat glass, for typical front lens material placed ahead of reflectors.
I have attempted to search here and about everywhere online, but I can’t pin anything down as far as specific thicknesses (from 1.5mm to 2.5mm will work), or sizes available for purchase that includes specifications on transmissivity.
If it makes the stuff any easier to source, I could cut it myself if anyone knows of good sources for rectangular sheets.
https://flashlightlens.com/glass-lenses/
they state light transmission, but It’s going to be high anyway with any AR coarted glass, it’s also very easy to test oneself with a luxmeter or just a phone (lux meter app like ceiling bounce), considering this, you can find AR coated glass windows on Kaidomain and Convoy aliexpress store in various sizes.
At flashlightlens(dot)com you can buy almost every diameter of flat glass lens. If thickness is a problem, you might add an o-ring between lens and reflector, or solder a suitable copper disk under the mcpcb.