Because these threads inevitably involve someone saying "or, you could do it THIS way", I'll throw in my two cents.
I don't like the position of a dash mounted outlet. I used the OEM one on one of my first big trips cross country, where I had a GPS mounted to the accessory rail. The GPS power cable had to be secured to something to keep it from flopping in front of the instrument cluster, and I definitely didn't like having a power cable going from the dash to a handlebar mount. I like the look of dash infill panels, but they also look like they'd complicate maintenance tasks like changing headlight bulbs (something I've had to do multiple times on this bike) or removing the steering head.
There are a lot of handlebar mounted USB outlets available that can provide a much more convenient location than the dash to plug in a handlebar mounted accessory, and that won't involve having to cut holes in your bodywork. Amazon has a bunch of them. Touratech sells (or used to sell) a ridiculously overpriced bracket that you could mount to one of the handlebar clamps and attach an outlet there. I went with that solution, though I made my own brackets out of a piece of 1/8" aluminum:
Rather than using a dedicated USB outlet, I use a standard 12 volt outlet with a USB adapter. My 12 volt outlets are rated for 15 amps and wired with 12 gauge wire, so they can safely be used to also power an air compressor if I have a flat. Having an outlet on the bike that can run everything from a 5 volt GPS pulling milliamps to a compressor running 12 volts at 10 amps can really come in handy.
For stuff mounted to the accessory rail, I wanted the outlets to be as close as possible to the rail to eliminate long power cable runs. I have two USB outlets mounted right above the instrument cluster, so the power cables for things like my GPS or phone only have to be about 6" long.
They sit on a shelf I made from an old ammo can. The shelf bolts to some unused holes in the windshield mount, and the USB outlets screw into the shelf.