Well I did mention the name: RAM Aquabox. that is just the box is is in. Not specifically for the n900, it has some room to spare. and it is not quite cheap. but quality is good and the mounting system seems solid enough.
The plastic front is quite thick but fortunately the touchscreen still works through it.
Link:
http://www.ram-mount.com/Products/Aq...5/Default.aspx
(I bought it online from the Dutch shop which is noticeably more expensive than the USA one. but the USA shop doesn't ship abroad.)

Edit:
I changed the "point reached distance" back to 30m ( 30 is the default distance?) as it switched to the next turn a little too fast for my liking while driving inside town. Also found out that you should wait for a accurate fix before you start driving because I drove off with my position showing next to the road instead of on it and it kept being off the whole time while moving.