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I have solved my problem in a needlessly expensive and complex fashion (I have time on my hands). I'm getting those dyna-snap magnetic connectors so I can remove the bulky switch from the set up when I'm not doing multi-day rides. I just need to bodge a velcro strap for the switch to replace the zip ties. And to make it more complex and time consuming I might try and get a new 3d printed case for the switch.
I’ve just used a standard 3 way toggle switch I got from Maplin (RIP) that was designed for small electronics projects. Wired up to lights and charger and currently attached to a small battery bag under my top tube. This goes to a usb hub rather than cache battery however.
I’ve also seen variations of this method mounted into a bar end, which looked really neat. Can’t find a photo now however, but I’m sure you get the idea.