Okay, got some ideas. Want to add this at times but remove at other times?
I was in need of some way to extend tubing as I have tanks near my reserve water as well as in other rooms. Maybe a use that could work for you? For a reliable system to take my fill tubing apart, I used CPVC pipe fittings. Plastic pipe, something like PVC. A union fitting can be screwed together by hand and is easy to unscrew when not needed.
This is a quick "top of the head" thought and can likely be improved if it sounds like the right track.
My setup lays out like this:
Tubing adapter to threaded fitting glued to small piece of pipe, glued to one portion of the union. The ring to tighten and the faucet washer for the seal. This then mates up with the second portion of the union.
Using unions if needed but go simple with a threaded cap if the entire tube end can be rotated enough to unscrew at a fitting?
Getting any where close to something that might work?
Brainstorms are funny things--- they can take time and they also explode at times! But most of them are very cheap to replace..