I use a kuda mount for my myfi. Here is a pic:
Here is the Kuda website
if you want to check it out. I am not sure how big the nav unit is. However, you can actually drill a hole behind the unit to run the wires completely out of site. I actually just tuck them under it.
It looks like they added a new dash mount for the E90 and I may need to get one of them, too. It would be a nice way to mount my Treo.