So, I was looking at bolt end covers after looking at a wire shelf end cap in my hardware cabinet, and a light bulb went off. It appears that the vinyl bold caps come in many sizes and depths. My tubes are mostly 3/8th inch (or 10mm) and there is an end cap that size. They come in a variety of depths as well, as they are designed to protect bolt ends from injuring others, or from paint when a bolted product is painted. Protects the threads.
I also found a 3/8 inch silicone end cap in the auto world, as vacuum line caps, so I ordered some of those as well. I am currently using both and will see which is better. I suspect the silicone will work better, as it it stretchier and more pliable, making a better seal. If one buys them in bulk, they are cheap. The green ones in the photos were 50 for 10$. If you get the 3/8 x 1/2" length vinyl caps from the hardware store, they are over 50c apiece. I suspect the silicone will be more resistant to paint adhesion as well, since I use silicone pans to mix gesso, and just squish the pan when the remnant is dry, and it just falls out or easily peels out. Nothing sticks to it....
Stay tuned, and I will see how they do.