They're glass with teflon-lined caps. After mixing five liters of XTOL (using distilled water), I fill 20 of the bottles to their tops, then cap securely. No oxygen is in or can enter the bottles. Each one constitutes a single film development run and is used in its entirety at one time. XTOL does develop a white, filimentary precipitate after a while, even when it was mixed using distilled water, but pouring through a coffee filter eliminates that very easily. I've used XTOL stored this way for a year with no difference between exposure or contrast indices of the resulting negatives compared to those developed with freshly mixed XTOL.
A one-time, lifetime investment. No squeezing, no gas, no marbles, no fuss, no muss.
I use the same idea with amber 4 Oz bottles. The caps I selected have a nipple in them to displace the few drops of Xtol at sealing. I bought mine from US Plastics. They actually hold about 4-1/2 Oz when filled so a bottle will deliver 9 Oz of 1:2, just right for 4, 4X5 negs (80 sq ") at a time.