I've got a fold up/slim voltmeter in my glovebox. I think it's something pretty much every 928 driver should have in there - along with the flashlight, of course. If we could get one with the 928OC brand, that would be cool. It closes up into a little card case about the same size a calculator. It's this one, actually:
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Not sure what it would take to get this logo'd... but I'll bet you'd get some takers.