I’m already up past my bedtime, so just a few pictures for now. Write-up coming soon!

Specs: Input 2v to 6v DC … Output constant current 50mA up to 28v DC
Efficiency: Initial measurements, somewhere around 75%
Chip Texas Instruments TPS61040

size comparison, american quarter dollar piece

circuit detail – design is one-sided PCB with two through-hole jumper wires and the diode, everything else is smt

twin 1uF tantalum capacitors … the output capacitor I had originally selected was limited to 16v, so this was the best I could come up with on a Sunday. Note the top of the coil is missing – these things are fragile!