I knew early on that I wanted the deck to use USB-C Power Delivery, as over the last couple of years I've switched most of my personal devices over to the standard. Luckily, so-called "trigger-modules" are easily sourced online that will handle the communication with the PD power supply.
Here I'm using the ZY12PDN, which happens to be an adjustable module. If I was going to do it over again I'd have simply selected a less expensive fixed-voltage mode, but at the time I wasn't sure how everything would fit together yet. If you're interested, the 3D printed holder for the trigger module is available on Thingiverse:
Though experimentation I found that 20 V seemed to be the most reliable, in that all the PD power supplies and cables I had could provide it. When I tried 9 or 15 V, sometimes nothing happened.
The 20 V coming from the trigger module then go into a 5 A adjustable step-down regulator that has been dialed into 5.25 V to provide USB nominal voltage. I should mention here that I went through a couple regulators before I found one that get anywhere near its promised specs. When tested under an adjustable load, the first ones I purchased all browned out at 2 - 3 amps.
From the regulator, the 5.25 V goes into a female barrel jack that's integrated into the bottom of the mounting block. This allows for the other end of the cable (which leads to the top side of the case) to be disconnected by opening up the former battery compartment door. This is demonstrated in the teardown video linked on the main project page.
I mentioned previously that there are no batteries onboard, though there's plenty of room underneath the keyboard should you want to go that route. Personally, I'm not much of a road warrior -- even my legitimate laptop spends 90% of the time plugged into the wall. That said, a USB-C PD battery bank capable of outputting 20 V should be able to run the deck.