What about using Argon. Its a noble gas and heavier than air and
readily available from welding supply stores. You can get one of the
small 20 cubic foot bottles and a cheap regulator with a flow meter for
a little over $100 if you shop around. You don't need to buy one of
those expensive industrial grade brass regulators (victor brand) you
can get a cheaper one for indoor use. The bottles are ususally
certified for about 10 years. The gas is very cheap. 20 cubic foot of
argon is equal to about 150 gallons of head space, so it should last.
Heres the trick set your flow regulator to 8 cubic feet per hour
(little floating ball) and every minuite is one gallon of gas that
comes out. I was thinking that if you had two gallons of head space do
you really need two gallons of argon? Remember its heavier than air
and will blanket the wine. Since its a noble gas it wont mix with any
other gas including oxygen, nitrogen ect. Its used in TIG welding to
blanket the melted metal to prevent oxidization. Its a little
investement up front though.

Hap