My airplane has a standard AC wall socket as a battery charging port (a 220V socket, with one blade turned sideways). I've got an adaptor using a matching plug to connect to the external charger.
This could be mounted flush to the skin, though best to use an offbeat socket like mine so folks don't try to recharge their phones at fly-ins.
However, it would be prone to pick up dirt and grease. I've got an unwired plug sticking in mine to keep the crud out. It's the round part at the top of this picture. The socket is surrounded by a rubber grommet, with the blank plug sticking out in the middle.
Ron Wanttaja