Kilonomicon will run primarily on 24VDC (lots and lots of 24VDC), but the controller boards will run on 5VDC (~20 mA typically, 250 mA worst case (if the RS485 bus shorts out)).  So, the plan is to convert the 24V to 5V at each board.  A linear regulator would be way too inefficient in this application, dissipating (24 - 5) x .02 = 380 mW.  

Fortunately, there are lots of switch mode power supply (SMPS) solutions out there, some with efficiencies exceeding 90%, from roll-your-own out of discrete parts to drop-in modules.  One of the newest of the latter is the TI LMZM23600 (that's it by the arrow in the photo below).  There are several neat things about this converter, not least of which is its tiny size.  For scale, the test point holes above and to the right (starting with "24V") are .1" apart.  As small as it is, it's a PCB containing two caps, inductor, transistor switch, and intelligence.  It'll convert up to 42V to 5V at up to 500 mA (there are also 3.3V and adjustable output versions).  All that for the low, low price of just $4.14, quantity 1.  Just add input and output caps (which you can see above and below the converter) and you're ready to go.

Unfortunately, like most SMPS solutions, it's least efficient at low power outputs.  In my case, that means most of the time it will be operating in the range of 40%.  It will still only be dissipating about 150 mW, less than half what the linear solution would, and won't even be warm to the touch.

I used solder paste and a heat gun to solder it, the caps, and the power connector to the PCB and it works as advertised.  Next up, the MCU, its associated passives, and a pushbutton and LED.