just got a 4x5 view camera, and within two days of ownership broke the glass. for less than $25 canadian I've got three make-work replacements, and I feel in short order I'll be able to make any number of format/application specific focus screens.
the screens I ground with 600 SiC look dimmer than the original Sinar piece when laid down on a piece of newspaper; if I stumble onto some AlO powder I shall try that as well.

amazing thread, thank you.