UPDATE: I improved the design to close some gaps. newest is FAT Doublestrike.stl
Instructions: Cut the notches in the front, dont cut everything down to the barrel, sand the cut area until it fits. Sand the 3 little bumps where the mod connects with the gun in the back on both sides.
I used a pressure fit, no glue needed but feel free to attach it how ever you want.
Scratching it makes it look nicer, files work amazing for that.
Print with the front part at the bottom.