There are two side supports and a cross brace in the back to hold it together. The parts are held together by some nuts and bolts on the back side.
In addition to providing the plans for my design, I will go over all the steps for you to design your own holder and cut it out, as you probably don't have the exact same model of GPS as I do.
I made this awesome piece at TechShop Pittsburgh using their laser cutters. They have an amazing variety of tools for all types of jobs and projects. Check them out at http://www.techshop.ws or stop in to your local TechShop location to take a tour. You can take classes and learn new skills and practice using powerful machines, and once you are a member you can use their facilities for your own projects.
3/4" #8 machine screws and matching nuts (4)
1/8" acrylic sheet or other suitable material
Calipers, or a good ruler
Drawing software, such as CorelDraw, AutoCAD, or Inkscape
Laser cutter, or a saw and very steady hands with a lot of patience :)