Hollow mast plug

All this talk about putting solid cores in hollow masts has me convinced I need to put a plug in mine.
 
No problem, cut one to fit on the table saw, put some kind of plug in it just above the partner, stand mast on end, climb ladder, pour in thickened epoxy, drive plug in.
 
The question becomes, what is the best way to plug the mast so the epoxy doesn't get wasted? Needs to be liquid tight, but easy to insert four feet into a hollow wooden tube.
 
Any suggestions from the collective?
 
Richard Spelling,http://www.spellingbusiness.com/boats
From the muddy waters of Oklahoma