As simple as an Eye terminal on your + battery terminal post(if you have an ON_OFF battery switch, go to that).

Attach the Eye to an inline fuse holder(hopefully that has 12 gauge wire), with a 20amp fuse. Run as short a length of 12 gauge marine wire as possible(if long, check the a voltage-length table), to the 'lighter' or USB receptacle. Connect neg back to an eye on your '-' battery terminal(or common ground bus).

Fasten the receptacle solidly. Nice, simple, safe.