This is a good idea, but can your application use plastic? I would suggest using lightweight PVC or ABS tubing, which is available as plumbing pipe at hardware stores or online at plastics suppliers with various colors, sizes, opacity, and densities. Also, plastic has a variety of tailor made fittings that can easily be shaped, glued, and painted, and is easily customizable for different sizes of battery pack. The CO2 cartridges are cheap, but hard to work with and difficult to thread because the hardness and thin cartridge walls. Furthermore, the cartridges are conductive while the plastic is not. If you must use a cartridge for the holder, I recommend using a rubber stopper or slip on cap, which is much easier than making a threaded cap.