Over the years I've tried many ways to seal these tube: nails, duct tape, cap, screws.etc
without success. Last year I began sealing them with a little plug of Plumber's Putty.
It helps to remove a little of the caulk, etc. from the tip of the tube. Next take a small piece
of putty and just stuff it into the tip. Put enough in to fill about 1/4 inch or more of the tip.
So far nothing's dried or hardened on tubes using this method and the putty can easily be
removed with a good squeeze on the caulk gun or by digging it out. Hope this work's for you.