When I did this, the only thing I could do was find an actual carry-handle Armalite upper (which was not cheap) to fit my Diamondback 80%. I still had to machine it to accept my magazines, and I still haven't completed the build.
I'm trying to decide what retro logo to engrave on my lower.
That said, Pic rail is all pic rail. If you have an FSB that's standard on your barrel, it won't match perfect but doesn't look terrible with an A2 carry handle on it.
The tight finger-hole area on a carry handle looks weird anyway, and would look even weirder if stretched longer.