I like the idea of using old golf balls, for easy mixing.
However, I use mostly 5 gal paint containers. So, if I have left over paint, I clean the rim and the lid, lay a piece of 6mm plastic, place the lid and hammer it around. When I want to use it again (in 6 months or less), I take it to the paint store and they shake it for me.
But Spruce, if you want me to shake your leftovers...it could be arranged.