I installed a BikeMaster USB charging port. It wires right to the battery. It comes with it's own fuse too. I used 2 black zip ties to attach it.
I wouldn't use it for extended charge times, as mentioned the battery has a small charge capacity, only 2 amp hours. But it's good for getting directions with your phone for a few minutes or so. If you do kill the battery, you can still do a push start to get the Reb running again.
Definitely DO NOT ride around with your phone plugged in. The Reb vibrates too much which makes any solid plug connections a no go. The result is a burned up electronics device. I found this out the hard way with a GPS.