Some of my friends buy wine in bulk packages, which consist of a mylar
bag (that mirror plastic) in a cardboard box with a polyethylene spigot.
They save these packages for me. The mylar seems to be totally airtight,
and the poly is certainly no worse than other containers which are
commercially available.
With care, you can pry off and replace the poly seals many times. Clean
out the bag with multiple rinses, and fill the bag while inverted, just
as full as you can get it. Replace the seal. Then, when you draw out
liquid, the bag collapses inside the box. No air gets back in. What
little air might have been contained in the bag initially will oxidize a
very small amount of the whole amount of initial filling; what gas
remains will be mostly nitrogen - inert.
The worst part of this method is that the boxes tend to get pretty awful
after a year or two. But the best part is that the chemicals last, oh
boy, do they! I have kept Type C developer, which tends to oxidize very
badly when stored, for - maybe you won't believe it, but it is true - as
much as a whole year and it has maintained its color and worked
perfectly. I have also kept various film developers, some containing
pyro (which is also a heavy oxidizer) for long periods of time.
These same bags are used by my kayaking friends as water storage devices
(not the same ones I use for pyro!); they make fabric slipcases for them.
They will stuff into odd spaces in the boat. In a pinch, they make a
passable pillow.
Give it a try. Everyone I have turned on to this just loves it.