I discovered you can get C123 sized lithium-ion batteries, so at 3.7V they are enough to drive a K155ID1 and puts the voltage safely over the 3V minimum for my HV power supply. They only have 700mAh compared with 1500mAh of the non-rechargeable C123 batteries, plus I measured the current for the complete circuit - it is around 130mA. So this should last around 5h 20m, which is good enough I think. Hopefully I can get the current draw down some - it all depends on the draw of the SOIC K155ID1 I will be using - still waiting for an adapter from OSHPark to try this out.

I have the PCB for the HV designed, and I know what I am going to be doing for the display board - I need to see if I can fit it on to a double-sided 18mm diameter PCB. I have to pull out a programming header for the ATTiny84 as I'll be using the 4mm QFN version, so I can't jusy use a ZIF socket for that.

The limiting factor on the housing will be the battery holder - the battery itself is 16mm diameter and I am waiting for a couple of different holders to arrive from DigiKey.