I made a GPS mount from a wire coat hanger. I wrote up instructions with photos here:

http://www.miniacal.com/garmin_mount/



It hooks into the vent.


There are some more pics at the link above.

 

Simple and elegant. I might suggest covering the wire with a layer or two of shrink-wrap (the same as used for wiring connections) to keep the metal wire from marring the plastic of the car (and likely quieter as well).

 

Or, dip it in some of the tool grip stuff that hardware stores sell....

 

Made another DIY Garmin Mount. Made from Aluminum, base is 2.717 diameter with a 1.5" stem thats roughly 1/2 diameter. Cut off the 17mm ball on the Garmin suction mount and used epoxy to glue onto the end of the 1/2 diameter stem. Drilled a hole thru the base to be able to use the charging cord. Will hard wire to the fuse box. Here are some pics.

 

A few more Pics: