Best mod I've seen was on a 97 GS1100 a double power socket which was mounted to the nearside of the headstock close to the fuel tank, using bolt holes already in the headstock casting.
The two female sockets,( standard Bosch tractor fitments and cheaply available from any tractor dealers), were mounted on a rectangular stainless bracket and wired to the battery direct with independently fused leads.
This location is much better than the standard position as it is well away from road dirt and reasonabley dry, it also allows for shorter power leads to be used on any plug in extras without trailling wires all over the bike.