Cartridge/armwand storage

I, like some of you, have an extra cartridge and armwand and haven't been able to think of a suitable way to store
it when I'm using the other one. What do some of you do?
Use a box made from acid free paper.

Store plastic/wood/paper packing materials/instructions separately from the cartridge as they may eventually outgas various nasties.

I store rubber parts, "O" rings, belts, etc., in such.

Photo shops and Art stores often carry stock (acid free boxes/paper).