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.
Step 2: Measure, Cut and Fold
Measure the top of your device from side to side. Center that measurement on the back of your plastic piece, and draw lines representing the outside edges of your device with your Sharpie for reference; you'll fold along these later to create the main part of the hood. I put my device on the plastic to trace, as that was fastest.
Draw an angled line from each of the Sharpie lines from the outer edge of your hood to the outside (what used to be the top and bottom of the case) edges of the DVD case; I left a tiny bit of edging there for strength, but it isn't really needed. You can see two notches in the sides of my cover that were where the booklet spring clips were.
Measure 1.5 Inches from the back edge of the plastic to cut out enough plastic to leave two connector flaps on either side. Again, I traced my GPS as it was quickest, and allowed me to match the rounded contour of my particular device.
Cut out all the outside edges of your design with the sharp implement with which you are A) most comfortable , and B) least likely to injure yourself. My lines are straight and basic- feel free to get fancier if you want something different.
Next, simply make four folds - two for the tabs, two for the hood. All go to the inside, as shown in my image. I used the measuring grid inside the plastic to help keep my lines crisp. You can see how my folds show stresslines from bending- you could minimize that by scoring the bends first, but I did mine without as I felt it would be stronger in the long run.
Step 3: Attaching the Hood
Cut two pieces each of hook and loop to fit the tabs on the hood. I used peel-and-stick Velcro, so I attached the hook side to the hood, with the loop sidestill attached, then pressed it to the GPS to simplify lining them up and get a perfect fit.
If you're happy with your cutting, you can stop right here!
Step 4: Bit of Polish...
I lightly sanded my edges with a nail board to round the sharp edges, then ran a black Sharpie around them to blacken them up.
Finally, I gave the hood it a weigh-in on the scale, and it's a tidy .75 of an ounce. This means it won't really add a significant amount of weight when I return it to my windshield mount.
Plus, 'cause I'm a geek, I added some dressing to make it more futuristic and hide the brand name.
Step 5: Test and Wrap Up
Light and Sturdy
Easy to make
It MIGHT look a little dorky. D:
I may now have to build one for my stereo headunit...