A friend asked me to design and print him a few caps to keep the dirt out of his CO2 gas refill devices - and to keep them from discharging while in his pocket. I took a few measurements of the device, which takes a standard 12g CO2 capsule and has a simple valve and pressure gauge, and designed this simple cap (with a few added dimples to make it look the part). The Cap has a hole for inserting paracord or similar (4mm diameter) to allow the cap to be tied to the charger device. There exist a number of versionf from various manufacturers, but the product this one was measured off of is this: https://airsoftpro.cz/en/airsoft-guns/sniper-rifles/other-grenades/co2-plnicka-airsoftovych-granatu-s-manometrem-detail