What is the diameter of the accessory plug?
I have the 3rd plug fitted to my Touratech air pump so I can use it on the bike and also in a car cigarette lighter socket if needed.ISO 4165
Comparison of (L to R) two Powerlet plugs, one combination plug, and one Cigar Lighter Plug
This double-pole DC connector is designed to supply between 12 and 24 V DC at up to 12 Amps to appliances in vehicles. Although roughly similar in design to an automotive accessory receptacle, the ISO 4165 connector is shorter and smaller in diameter. It was originally a standard fitting on all the German military vehicles during World War II.
The body (which remains outside the receptacle) is 20 mm in diameter, the resilient neck must mate with a 12 mm diameter hole, and the center pin is 5 mm in diameter.
The ISO 4165:2001  DC connector is also known as the BMW Accessory plug, as it is used on BMW motorcycles, the Powerlet connector, after of a firm that produces them, and, in Australia (where they are popular in mobile power applications, including 4WD and Caravaning), as the Hella plug, or Merit plug.
At least one manufacturer sells a plug that combines a cigarette lighter plug and a powerlet, in much the same way that combination auto/airplane notebook computer power adapters combine a cigarette lighter plug and an ARINC 628/EmPower plug.