Jim
Well any of the type plugs/sockets I mention would work and also have flush-mount sockets available as well as cord-end plugs and sockets.
Of course, if you use an oddball plug, the socket is then dedicated to that use unless you make a pigtail adapter to a standard cigarette-lighter socket that you can plug in something "standard."
The NEMA L1-15 plugs are about the diameter of a quarter (below is a 3 pole (center ground) -- you obviously need only a 2 pole (same dimensions.)
Another option smaller option is an 2-pole Amphenol type connector (last 2 pics) - about the diameter of a nickle. Their cases screw together nicely and are weathertight (I don't have a pic of the female half, it's 700 miles away right now.)
It really just depends on how secure you want it - push together like the 12v, 2-pole (trailer type) plug/socket, or a twist-lock or screw-together.