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Try the version of the cap I just put up with a clip. I make no guarantees that the thread won't strip out or come undone due to vibration, or the clip break off. Let me know if you have any suggestions.

The easiest way to add text would be to use Fusion 360, which is free for non-commercial use. If you decide to go that route, I can walk you through the process. Otherwise, you can try searching youtube for "Meshmixer add text" and that will turn up some helpful videos using Meshmixer. I suggest you emboss any text otherwise printing will become very difficult.