JL Sleather is right. Where as other leather artisans wet form or wet mold, holster makers block and bone. You can’t block against something that isn’t there. For example, on Beretta M9’s they have an open frame top. But we want something hard to block against to create the sight track. I’ll put packing tape on the area, fill in the open space with hot glue, then form the sight track. Then I’ll cover the glue with tape. Now when I bock and bone the area, it stays blocked since it had something to block against. Then pull off the track, and save it for the next Beretta. In your case you could fill the rail voids, then tape and block/bone. Now days I supposed you could laser scan a sight area or rail and 3D print a block filler. Good thing is your boning tight. Set a close stitch line, bone tight, and the piece will break draw off the barrel without boning into the rail sections.