Fentible,
Sorry I missed you original post, but for what it's worth I have just upgraded my bike GPS from a car Tom Tom in a water-resistant housing (identical to yours) to a proper bike variant - Garmin Zumo 590LM and spent last weekend fitting it to Heidi.
Main reason for the change was because I wanted the bluetooth connectivity (the car tomtom blue tooth will not talk to my blue tooth helmet comms set) and for a few other features that came with the Zumo - the tomtom in a bag had worked very well for me, albeit slightly bulky, and was a considerably cheaper option than a proper bike version (even with the current £137 discount that Garmin are offering).
Anyway, back to the mount....
I used to have the GPS-in-a-bag mounted on the dash - but you don't have that luxury with your naked girl
I had the same dilemma as you - where to stick the new one and how to stick it there.
The Zumo came with a RAM handlebar mount - but there is just no spare handlebar free, unless you are happy with it being offset to one side. Personally, I didn't like the offset look - and please don't take offence (cos none is intended), I think it spoils the sleek lines of your bike.
I had the option to mount it on top of the dash roughly where my old one was - I didn't like it there as, although it gave it a good heads up viewing position, it obscured the fuel & temp gauges and the top of the instrument cluster (most importantly hides the indicator flashers). The second option was to attach the RAM mount to the rubber ignition/switch dash in the centre. of the handlebars. This was the option I went for.
Having decided on the location, it was then a simple matter of carefully drilling two 6mm holes through the rubber, in-between the heated grip and ABS switches (centre two of the 4 switches/dummy switches). I then passed two M6x70mm bolts through these holes (these are a little long and will get cropped in due course - 60mm would be long enough, but I couldn't get that size easily) with washers between the bolt head and the rubber mount to prevent the bolt pulling through. The RAM mount was then positioned over the bolts and secured with nuts (I will change these to nyloc nuts when I get to the shop!). The resulting set up (with the RAM mount assembled) has the GPS overhanging the fuel tank (no good if you use a tank bag, but probably not an issue on a naked bike).
The switches all remain accessible and....so far the set up seems to work well. The GPS is within easy reach and easily visible (mine is hardwired into the bike electrics using the supplied bike mount).
RAM mounts are available in a wide selection of options to suit many applications:
http://www.ram-mount.co.uk