This is the method I have come up with to use a GPS intended for cars on a bike.
I bought the case from this
company but I have to admit that it was only £14.99 when I bought it. I think it must have been a mistake as it is now £124.99!!!!
I houses a Tom Tom 730 with full European maps. I have used it to check everything works but I have not used it for anything other than testing trips so far. We are heading off for two weeks into mainland Europe in August so I can let you know a full report afterwards.
Here are a few pictures showing how it mounts etc.


mounted on the original headstock nut with a Ram Mount fitting (uses UNF nut - not supplied).

Case with Ram Mount fitted to rear.
It looks big, but bear in mind this is an SV650. You can reach the ignition key easily and the clocks are not obscured at all from the riding position. The Ram Mount does offer adjustment forwards and backwards. As it is it clears the Bagster tank bag easily.
I had to make some small modifications to allow for cable access. The lump that can be seen on the left of the case in other pictures is for cable access and has all the seals needed to stop water ingress. However, I completely forgot about the fact that the Tom Tom needs to have the cables fed in from the bottom, not the side as the PDAs do that this case was originally developed for do. My own fault and I kick myself for it a little
When in position the water should not be a problem as it will just drip down in the still pocket of air... I hope
The leads are for power (needed a cheap micros USB lead because the Tom Tom original was 90 defree item) and iPod connection along with Audio that plugs into my AutoCom.
The front surface it flexible and you can control the Tom Tom when inside the case. I have found that even when outside the case a fat finger can give problems, but a small point similar to a PDA stylus works perfectly. I may add one on a cord to the case so that zooming the map to check details of the route is easier. But in testing I have not found it a problem as the route will be planned beforehand.
All in all I think it will work well as I have used Tom Tom on the phone linked to the intercom, but the problem came when in and around large junctions etc. The ability to look at the screen is a big help and also just reasures that all is well when the thing has been quiet for 10 minutes. Also I can control my iPod fromt he screen.
Cheers,
Simon