I've seen several options for carrying spares.
1 Check out the
spare carrier from Surefire. It's heavy plastic with a seal & will hold four batteries. They also make a six cell model
2
A Pelican case with foam 3
Battery Holder 4
Unsealed CaseFor times where getting it wet isn't a factor, I'm using options three & four.
When water proof is needed, the spares carriers are more compact but carry fewer cells.