The file '12mm John Guest Tubing Cutting Jig 3D Printer Model' is (stl) file type, size is 7.3KB.
My Swift motorhome has 12mm OD pipes so I've simply upscaled the 8mm jig in from jrandombob and tested it fine with the 12mm tubing.
Printed with 100% infill for strength.
12mm_JG_Tubing_Jig.stl | 34.4KB |