Here's what ALL of my ARs have, Go to Sportsmans guide and look up part # WX2-90427, they call it a "tower rail mount", it gos on the FSB, they are only $15ish, I can vouch for the fit and quality, just use a little blue locktight on the screw, for good measure.
I bought a G2, but I didn't like the twist on/off tail cap, now I have about a dozen Brinkman "maxfire LX" lights, they use a push putton tail cap, I have tested them under live fire on ARs and 12 guage shotguns, not a problem yet. The light gos for $20 at Wal-mart, or $15 @ Target, it is the same size as the G2, uses lithium batterys and all, I buy a dozen SureFire batterys from Cheaper Than Dirt, about $18 (IIRC), way cheaper than I could find local, about $7 each at Wal-Mart. I just go to the store and get a 1" scope ring (come in 2 packs). Works absolutly perfect.
ETA: TOTAL COST APPROX. $35 TO $50(AND WORTH EVERY PENNY)
Good luck man. I have blown a lot of money on a lot of shit (acceserys) but a light is the only thing I have every left on and never got tired of it, I think if I could have only one accessery, it would be the light.