I doubt I am the first to do this but it may come in useful to others. Having used a just a small amount from a tube of sealant (gun type) I was looking for something to cap the end off neatly rather than leave nozzel on to form plug or sticking something up the nozzle. Found that the thread is the same or very close to the toothpaste that was nearby. A Colgate one in this case ,has a little flip up cap . Fitted the sealant tube a treat and if I want just a small blob of anything in similar tubes in future I can just flip open the cap and quickly close it again. Won't be throwing away toothpaste caps for a while.
G.Harman