If that mount has a RAM mount fitting, then the best thing to buy is a Teleferizer RAM mount for the S
http://www.telferizer.com/
Takes about 10mins to fit. If that mount doesn't have a RAM fitting, then I'm sure there must be one about, have a look in the links on the Telferizer site. £27
I've got a Telferizer, its great.
If not one of these, but its probably the wrong shape
http://www.hein-gericke.co.uk/street...ps-tasche.html