This isn't a GPS but it could get the mental juices flowing. This is my portable satellite radio. I bent up a piece of aluminum that I bought from the Home depot and bent and shaped it with a little propane torch and a bench vise and metal file. I bought a longer bolt and put 3 flat stainless steel washers underneath it so it wasn't crushing the plastic. This allows you to tighten it down really well. I plan on eventually replacing the plastic washer on top with a stainless one but haven't gotten around to it. I attached a belt clip so that I could remove the radio really easily. Having the bend and the leather case really absorbs the vibrations and have had no problems as of yet.
I plan on attaching a ram ball to the adjacent hole and mounting my Garmin. I haven't found a ram ball that has the same hole diameter but I have seen ram balls that mount into a flat base and have 2 mounting holes. In that case, all that would be required is a longer bolt and a long ram ball arm to get it high enough to be in view.
Hope that helps