Suggestions for 3d printing caps

[Da9L]

I got myself a 3D printer some weeks ago and it just hit me. Couldn't i use this thing to print me a top cap that is not just for single use ? I mainly think of the Lava Lite lamps, and specifically the Grandes, as the Mathmos' lamps i have, that I've had to take the cap off of, are screw caps and therefore no problem to put back on. The lava lites on the other hand is not.

So i was wondering if anyone have had any thoughts about a possible way to seal Grandes in a way like this ? I could propably create a 3D model that others could print, so that it would also benefit others!