Option #1. Google AR15 two part handgaurd and you're on your way. Lot's of different models to choose from and you leave the A2 sight in place.
Option #2 cut off the top of the A2 sight leaving the gas block portion in place, slide on a one piece tube, add flip up front sight.
Option #3 is my preference, and what I've done before:
Remove A2 sight, install whatever free float handguard you like, then re-install A2 sight. The handguard has to be short enough to clear the sight of course, but does not have to be a 2-piece. No need to modify the sight that way.
I've used a number of the cheaper Matrix handguards for stuff like this. IIRC I used a 9" on the last one, and had to trim about 1/4" off the top front of the rail to clear the sight but it looked good when done.
