I recommended a great inexpensive light in your other thread. Don't take my word for it, look at some of the other reviews. I have a lot of experience with LED's (24hr MTB racer) and this little light is solid. It comes with 2 solid aluminum bodies, picitinny rail mount and remote for $50. Bright Cree emitter too.
Since you don't want the shell holder but want a rail and place to attach a light, I'd suggest you buy the rail system from sidearmor and attach a light on the side, thus not obscuring your sights.