I just installed one in my $89 Stag lower, with outstanding results. Used an un-modified DPMS carrier. Also had to shorten the top of the paddle a bit
There was a minimum of dremmeling involved (to the lower) not difficult - and later, and I did square up the back of the carrier. It beat having to shell out $$ for an SP1
Posted some pics of how I accomplished it, on a different thread / here:
http://www.quarterbore.net/forums/showthread.php?t=959
Steve in EL Paso