I completed the permanent installation of the 12 volt converter on my bike. The converter had been on there for a couple months, but was left unplugged because I didn't want the battery to drain all the time the bike was turned on. I knew it needed a switch, but I couldn't find a suitable mount for it. I recently found a mount design I liked, and 3D printed it.
The switch cuts power to the converter, and to turn it on I just reach under the edge of the rack and flip the switch forward. On my bike I use XT90 connectors for main power lines, XT60 for high voltage accessories (52v), and small Anderson connectors for low voltage (12v). The first photo is the switch mount still on the printer, second photo shows the rack removed from the bike and upside-down. You can get an idea of the orientation by the taillight at the top of the photo. The actual 12 volt output with the Anderson connectors is covered with a red cap which is actually made for an XT connector, but it fits, keeps it clean, and prevents it from rattling ;)
When the rack is on the bike, the top of the fender is almost touching the switch mount and converter, so the Anderson connector stays in this position nicely. For connecting to a 12v accessory, just I pull off the red cap and plug the accessory in I currently have 3 accessories I use with the system: One is a simple 12 volt power outlet, similar to the one in most cars, and you can plug pretty much anything into it. The second accessory is a high power USB strip with 6 USB-A fast charging ports (up to 13 amps). The third accessory is a a direct-connected 110 volt AC power inverter with a 350 watt output, and this can actually charge other e-bikes.
If you have any questions or suggestions for improvement, let me know!
Genius! Looks like a clean install too.