15 days to get here from JLCPCB, and that was with the cheaper shipping option.  When I got home, I pulled the parts off the breadboard, stuffed them into one of the PCBs (panelized 3-up, so I received 15 total), and hooked it inline between a Ryobi NiCd battery pack and a buck converter...and it worked!

Now I need to work on the packaging to turn all of this into a durable adapter that I can bring on the road with some battery packs...might need to start another project for that.