Gameboy Zero Battery holder
thingiverse
A basic enclosure for a lipo battery, it's been some time so have no recollection what size battery was used. It is a straightforward square holder with a notch cut out. If this one doesn't work for you I'm certain you could knock one up in mere minutes using Fusion 360. If memory serves me correctly, the ridges on the back of the battery cover were ground down to provide a bit more room.
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