Oh, I've been looking. Cygolite makes a whole lotta lights, it's not outside the realm of possibility that they had these batteries batch made. I found one Li-ion that was close in dimensions, but has a 2-wire lead.

That one is too long because it's the explodey LiPo kind, and has to have the regulator board on the end.

I have the battery from my original Hotshot 2W, which has a broken pin in the mini-USB port-- so it can't be charged. While I have one "extra" battery, finding a source for replacements would be great.