You are correct good sir , TM haven’t made a fixed upper for the M4 recoils but it is possible to do , there’s been a couple of documented builds on different forums over the yrs . I actually built one my self a good while ago , basically I cut away the lower side rail on the carrying handle , drilled and tapped a couple of holes in the base corresponding to ris rail gaps thread locked a couple of pieces of threaded rod in to the holes then epoxy putty in the rail gaps pushed the the rods in to the putty , filled all the side gaps in then sanded and painted then in a Blackhawk down themed camo and then age it a bit .👍