Marty, you might be able to overcome the airlock leak issue by filling
it w/ glycerin, but using inert gas seems like a lot of work/expense
for that volume. keeping different sized containers on hand will solve
your problem cost effectively, esp b/c the smaller containers are
either free or inexpensive: 750ml wine bottle (free), 1.5L wine bottle
(free), 1 gal jug ($4), 3 gal carboy ($10). Another low-cost solution
is to save your used oak cubes (not chips), and store them in a strong
liquid sulfite solution. When you need to top up, rinse the cubes off
and throw them in the carboy...remember to separate cubes used in white
and red wines.

-David
Denver, CO USA