The housing was always going to be some kind of tube. The question was, what material and how big? The limiting factory here is the battery holder. I bough the most compact holder I could for a C123 battery and then measured it when the battery was in. It turns out to be just slightly bigger than the inside diameter of 3/4" copper pipe, but that is because it has a fkat bottom designed to be fit flush on a PCB. I figured I could file the corners off.

So I bought a length of 3/4" copper pipe and filed the corners off and yes, it fits. I cut a piece to roughly the right length, inserted the battery + holder and got a quick lesson in the dangers of doing this, and just how low the internal resistance of a LiPo battery is. The are a couple of pins coming off the bottom of the battery holder, and the copper tube neatly shorted them out. Lot's of glowing, very warm tube and a slightly warm LiPo battery.

Anyway, I snipped the pins short so they don't contact the tube now:

Next up is figuring out how to secure the PCBs in there in such a way that I can remove them if I want to.

Interestingly, shorting out the HV is less of a problem, not just because the PCB design keeps the HV away from the edges, but also because the circuit will shut down if it detects a short.