There's a place called crankyfarmer.com that has one that uses two screws in the back to lock the cover so it can't rock and affect POI. If I were going to try one, it'd probably be that one.
What I want to find is an inexpensive handguard or gas tube with a rail so I can mount a red-dot sight on it. I know Ultimak makes a good one, but I'm cheap, I hate to lay out a hundred dollars for it. Especially not knowing what difference(if any) it would make in accuracy.