I got myself one of these
Eriding Dual USB Charger 5V 2.1A with Din Hella Socket ready to go USB adapters for the power socket, and it works just fine. There are a few other pre-wired options on the same product page.
But my goal was not to permanently mount it on the bike, but rather to have it available for when I ride with the Insta360 OneX camera, since the batteries on that last for about an hour and there's room for a lot more video. I've zip tied the USB plug to one of the RAM extensions that I use to hold the camera, and I kind of thread the wire through the frame on the ride. I should probably get some velcro straps or something to make it more secure, but it works, and when I'm home everything comes off cleanly.