I've been using this design for years with my camera batteries. The original was for the NP-FW50s (which you can find HERE). Now I've updated it to work with the NP-FZ100s :D

The side dovetails let you securely combine a number of them together. There's a sloped back, counter to a retaining edge on the front top that keeps the batteries from unintentionally sliding out. The flat, side-by-side arrangement allows them to sit comfortably in a side pocket.

The design works best in PETG, which allows for a little flexibility, and good all around durability. The walls are 1.2mm thick, to be printed with 3 perimeters using a 0.4nz, or 2 perimeters with a 0.6nz.