I use a media bag from Nippy Normans, you also need the mounting plate then can use standard ram mounts with it so you don't need to buy the multipod, works great. Use it with my TT One original.
I use my Tom Tom 720 on my GS with a Ram Mount and it works excellent, all I do to waterproof is carry a collection of small sandwich bags and use them to cover and then attach to the mount with the power cable thru bottom of bag. Not perfect but so far so good. Think it was Ram Man on this forum helped with the acquisition of the mount.
This looks the same one http://www.buybits.com/product/sku_2999.aspx