AAA, AA, C, D, or 18650 Battery Cap

thingiverse

I needed a compact battery cap for spare AAA batteries stashed in my bag, so I reworked my existing battery holder into an adapter for AAA batteries. Next, I scaled up the design to accommodate other sizes. The design is deliberately tight with minimal clearance because it requires post-processing to achieve a snug fit when using rigid filaments like PLA. To smooth out imperfections and get the desired fit on a small hole, wrap sandpaper around a suitable bolt, secure it with duct tape, and rotate it inside the cap. For larger holes, use a sharp knife for cleaning. Flexible filament might work well too, but I haven't tested it yet. This design works in Customizer. Batteries from different manufacturers vary slightly in size, which may necessitate adjustments to the dimensions.