If you are wanting to fill in the slots I assume you will not be attaching anything to them, correct? If so, just mill off the slots and top completely? I’ve had Paladin Machine Shop Services in Kershaw SC mill off most of the Pic rail on the so I only had the slots at the ends, like MI light rails. They did not offer light rails on the .308 forend so I had Paladin turn their regular fully slotted top rail into what I wanted.