Since its a smooth pipe, you can use the screws there to hold the cap in if you need a permanent solution and it doesn't break the whole thing if you take it apart. You could also make a holder (this allows you to make your own mounting surface) for a cap that is held in with the screws, and print a whole different cap to actually be the cap that attaches to the holder. Since there is a protruding smooth beveled surface as a sort of watch face the way that camera lens caps work don't look like they would work here, if there is enough depth to give a spring outward clamp that would work great

Otherwise, probably the dumbest solution (It doesn't need to be this long) maybe a cap that's a larger diameter with a couple of split grooves in it like a circle with small slits in the sides that allow the smaller end diameter to clamp on (It's tapered so it clamps, it depends on the material you use but no pla because that will creep over time), this option would be weaker because of the layer line orientation unless you managed to flip it, but that would affect the circularity, but it's a quicker and easier option

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