What is it you are trying to do? For example, do you want the socket flush mounted or are you happy to just have it mounted on a holder?
You have a choice....
A. A cigarette lighter plug and socket, as found on most cars. These sockets sometimes come in two separate lengths, depending on how much space you have available.
B. A Hela plug and socket, as found on say, most BMW motorcycles.
Both are available in a waterproof form, pretty cheaply. The main difference between the sockets of both types is the caps. Some have a sprung loaded cap, whilst some have the sort of cap you’d find on an old film canister. The latter is more common on a cigarette lighter socket, simply because its outer diameter is larger.