I like the way you think
Dang Randy nice panel and that is a good idea. I kind of had the same deal with a Garmin 195. It is nice to have something to brace you hand on while pushing buttons.
Kent I emailed you several Pictures of Garmins mounted in a RV6/7/9 panel. Think there may be some ideas to consider.
I guess the obvious is one of the RAM mounts:
http://www.ram-mount.com/
The nice thing about a mobile GPS is taking it with you and using it in car, motorcycle, plane, boat or even hiking. I like the RAM products. A little expensive but worth it. Think out of the BOX. What about off the left side or center low, below the throttle/mix/prop.
If you want it IN the PANEL, than consider putting it where you can reach the buttons. With hand on stick and using right hand consider the reach and not blocking the screen while you pushing the buttons. You will use the buttons a fair amount, probably as much or more than any panel item, so give that priority.
The down side of cutting a big hole in the panel is its a big odd shaped hole. If you change to a new GPS you now have to cover or re-do the old cutout. You should consider surface mount, ON TOP of the panel. (see pics I emailed you)
Obviously its part function, part aesthetics and part limits in space and substructure. This is the fun part; you get to decide.