I use a jotto desk. Check out the "computer mounts" section at http://www.jottodesk.com/ They have quite a few different models and might even have one made specifically for your vehicle. I got the universal model one that bolts down under the passenger seat so that I didn't have to drill any permanent holes. Delorme (www.delorme.com) is also a distributor...I got mine for about $150.
My only complaint is that it wobbles a bit while driving but this can be taken care of with a good install.