I wanted a case to carry the 16Ah battery or a spare battery for my Magnum Premium folding bike. I considered the Pelican iM2306 dry box that would probably work (~$100) and also considered using a 10 liter dry bag. After searching for the case size and for cases for similar size tools, I started looking at cases for long range rifle scopes which lead to cases for reciprocating saws, both of which are a similar size. The one pictured below (Hermitshell Hard EVA Travel Case) seemed about the right size and it has foam inserts. The Magnum 16Ah battery fits perfectly with one foam insert. There is even room for the charger and cords as shown in the photo.