I just subscribed to Handyman magazine and a reader sent in a hot tip
about this.

He just screws a wire nut on the end of the tube. The kind that are
plastic caps with coiled wire inside.

http://www.aikencolon.com/Ideal-Wire...Nut_c_648.html

I've found plastic inserts at Harbor Freight, but unless they are on
sale at the local store, the price is outrageous considering that it's
just a plastic stick.

http://www.harborfreight.com/caulk-saver-97561.html