A simple cap that screws on the threads of common CO2 cartridges (either 1/2" x 20TPI UNF or 3/8" x 24TPI UNF) to protect the threads or adjacent items form damage when the cartridges are packed in a bag, carried in a pocket etc. The 1/2 inch variant works on UML life vest / PFD cartridges.

Print upside down, i.e. standing on the cap top, without supports. I used PETG.