We've added a cigarette lighter/power take off to our positive ground Austin-Healey 3000, and the same approach could be used for our BugEye: remove the grab handle on the passenger side and replace the piece of wood on the backing side of the instrument panel with a larger chunk of plywood. Then mount your USB/Power Take-Off/Cigarette Lighter onto the wood.
Wire it in as you choose to do so (and follow the earlier advice and ensure that you've included a fuse). And (to be traditional British), Bob's Your Uncle.
The wooden mounting eliminates concerns about positive/negative, and the fuse protects everything.
On our BJ7 Healey, we have a Garmin GPS unit held onto the heater box with rare earth magnets (bought ours from Lee Valley Tool, the Garmin is primarily to provide a GPS based speed.). But we can unplug it and use it to charge phones etc. Get yourself something that provides dual USB connectors. And Bob will Definitely be Your Uncle. Doug