I can only buy pineapple or cranberry juice in 1L containers, and even a cosmopilotian requires only 30mL of cranberry juice, so I end up throwing most of the juice away.
Does anyone else have this problem, and how do you get around it?
Small storage containers, freeze.
Being water-based, juice freezes very well.
This.
And this.
But if at all possible, just have the fruit lying around. Lemons, limes, and oranges are standard we-always-have-them purchases Chez Athena, for both cooking and cocktails. Tangerines and grapefruit are purchased on occasion. Juicing them with a hand-juicer is plenty good for cocktails, no need for fancy equipment.
Obviously, you’re not going to be making your own cranberry juice, but even pineapple and melon juice can be thrown together in a blender easily enough. Most of the time, though, I use the small cans for that kind of juice. Not as good, but even I can’t claim to have pineapples lying around all the time. When I do, though, the cocktails are divine.
Freeze in an ice cube tray but you do lose the Vitamin C, not that should be a big drama though.