Summary

I wanted a AA to C-cell battery adapter so I could use AAs in a pinch when we're out of C cells. I found a couple but they didn't seem to fit very well, which may just mean they want more flexible filaments than basic PLA, or maybe something to do with my rather basic printer, but I saw others making similar complaints.

So I took one of those (referenced) and made the opening for the AA cell bigger, and made the outer shell smaller, to more closely fit the C-cell official size specification. It's a little smaller than strict definition (according to Wikipedia) but it works well in my devices, and still maintains two lines in the outer wall with a 0.4mm nozzle, which felt important for strength purposes.

It's much better to use actual C-cell batteries most of the time - they'll last so much longer - but if you're in a weird situation where you never need C cells except SUDDENLY YOU DO, well, here y'go.