I believe what you are looking for is a Surefire FM37 beam protective cover. Surefire has discontinued production of it but you can find it in stock if you look around. Take a look here. http://www.lapolicegear.com/sufmbe5.html
This works great on the G2 metal bezels. If you have a G2 plastic bezel I would caution you to be very careful when installing it. The earlier plastic G2 bezels are a little larger in diameter than the 1.25" making for a VERY tight fit. I had a protective cover clamp break off from too much pressure due to the larger plastic diameter bezel. Surefire did NOT cover it under warranty in my case.