Print with .2mm layer height in spiral vase mode, no top or bottom layer. The spiral acts as threads for the couplers to screw together. Print with plastic of choice. I used AllProfessional3D Gunmetal PLA. If you want to go full "ninja" on this, print it in PVA so after you make the kill shot you can toss it in a rain gutter and it will dissolve the evidence.