Found this topic and researched for a good solution. Besides the above awesome solution(s) , found lots of these...
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Some with a bracket and some without.
Generally they are called... 12V to 5V Waterproof Motorcycle Mobile Phone USB Charger Power Adapter. Found them in all ranges , 1 or 1.5 or 2.1 AMP , even one that had duel USB slots of 1 and 2.1 AMP.
Yes you would have to use the 'fuse tap' method to tap into fuse panel , or your own fuse to another power source. I plan to 'fuse tap' into the heated seats fuse, which only power when 'key on'.