The UK/US adapter is the way to go for the mechanical connection for in seat power. They are cheap (10-15 bucks). Between the thicker prongs and grounding prong they are very secure in the in seat power connector. I got one with two USB A slots and a standard U.S. plug receptacle.

The in seat power is only rated for 75 watts and then it shuts down. In actual practice some chargers (transformers) at start up actually temporarily draw more current (inrush current) and will trip the breaker for the outlet. If it's close, you can try plugging the brick in, quickly removing and plugging back in (like 1/2 second between cycles) and SOMETIMES this will work (tested with a Anker 65W brick).

I am comfortable to say a 100W brick won't work on an AA seat power.

Hope this helps!