http://www.advrider.com/forums/showthread.php?t=790384
This one comes up regularly.
I replaced mine with a bit of 3mm Lexan about 3 years ago.
Still there.
Break out all the old glass, remove the rim, cut out Lexan, glue in with black Sikaflex, replace rim, mask the lexan in line with the inside edge of the rim, fill void with Sikaflex.
Job's done.
You can see the 3 year old job in this pic