Or if you don't want to drill holes in your bike somewhere, just get a normal socket and wire it up under the seat. Around $2.50 at jaycar.
Terminals for battery connection, crimp to wire, inline fuse, wire to socket. I wired a small on/off switch on mine too.
Or do a proper job using a relay wired to an ignition fed line like the rear tail light.
Use enough wire to let you bring the socket out to a tankbag. Power your GPS/phone from that using it's own usb cable. Simples!