I merged a shotgun shell holder and a picatinny rail to make this design remix. There are tow versions one with wedge that adds some resistance to the shell, and one with out. This way made in 3DBuilder by cutting off the top of the rail, and subtracting and merging it with the shell holder.

Credit: Shotgun shell holder made by Anticolette: https://www.thingiverse.com/thing:2952114 Picatinny Rail made by Fmascare: https://www.thingiverse.com/thing:3060697

Printing: Print shell holder vertically (shell standing up), print the clamp piece laying down. Thick wall, some infill. Check original creators for their recommendations.

Legal: The licenses are CC0 1.0 Deed for the shell holder, and CC BY 4.0 Deed for the rail. Act as if this new variant is CC BY 4.0 with credit to Anticolette, Mellonsmaps, and even if not required Fmascare.