I originally came up with this design for my Sony NP-FW50 batteries but had a friend recently mention that they'd work great for 9v batteries too. 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.

In my experience, they work best printed in PETG.