It works well for me Don, but I use it in combination with a piece of that stick on shade material that I place in whatever area of my window that needs it most. As some others do, I typically fly with one window completely down and the other up over my head. For me, it provides just the right balance between fun and practicality.
If you make something similar to mine, I might suggest that you use sew on Velcro for your material. I think you will get better service out of it that way, especially if it's taken on and off often. You will obviously need the stick on Velcro for application to your GPS. I'm not sure it's that important, but I also used a material that was flat black on both sides to minimize reflection. Good luck with it, Bill Don asked about the homemade GPS sun shield: How well does it work for you?
