I tried the soda bottle concept and I did not like it.

When you squeeze the bottle, you never manage to get all of the air out (you can verify it by turning the bottle upside down and seeing the air bubbles coming up to the bottom of the bottle).

I also bought a bottle of Kapri Sonne (which is a folding aluminium bottle like the one on my first post) and I noticed the same phenomenon.

So I came up with this idea: I will buy a transparent squeeze bottle with a long thin spout. I will fill it with one dose of developer (250 ml for me) and then squeeze the bottle.

Since the bottle is transparent, I will see exactly when the liquid goes up to the top of the spout, and then cap it.

That should keep a really minimal amount of air in the bottle and let me keep the developer for the full 6 months of the "fully stoppered bottle" described by Kodak.