The VTAC I was talking about is a one piece design as is the Lite rail.
After a little research it appears that a JP Vtac carbine tube with the tube extension would do the trick for you. https://www.m4carbine.net/showthread.php?t=78778
Wouldn’t take too much machining to cut a DD MFR for the sight and an access for the FSB pins. I have no clue how much strength you would lose, though.
Machine the DD to do what you want? why not?
Well, basically that’s going to take a lot of work to get on imho. It’s doable, but will take some problem solving.
The Vtac/JP does seem like the best bet though.