I'm afraid I thought of doing this as an Instructable after-the-fact, but figured it'd be a nice simple one to get my feet wet.
1 Standard DVD Case, black
3 in Velcro (black or color to match your GPS
1 ft Black Fabric Tape (Optional)
Metal or other sturdy ruler (to measure and to crease)
Sharpies (of a color to be seen against the DVD case)
Nail/Emery board/Sandpaper (Optional - if you want to smooth out the edges)
File folder and pencil (Optional - only if you want to make a test pattern beforehand)
Approximately :30 minutes, 1 Hr if you're a perfectionist.
Step 1: Designing the hood
I managed to do this without creating a pattern, but you might want to if you're worried about wasting plastic.
Remove and discard the clear plastic sleeve (and the paper insert if it is still in the sleeve.) I'm condensing all of my DVDs to slimline packaging, so I have a bajillion old-style boxes I haven't recycled yet.
Cut and remove the disc-holder panel, and "spine" piece, leaving only the front panel.
Using scissors (or X-acto knife) CAREFULLY cut inside the case and remove the "lip" from around the edge of the front panel, leaving a nice tidy sheet of plastic with two holes where the "booklet" spring clips were located.