This cap protects the threads on a CO2 bottle from accidental drops. I have a couple of aluminum ones that I got from a paintball shop and I have found them very useful.
It can also be used to identify if a bottle is full or empty, either by using different colors or by using the files with an "F" or "E" on them.
Mine printed best at 0.15mm layers, 50% infill and two perimeters. Drill the holes at the top through so that no pressure can build up under the cap.