I decided to take this idea one step further. Went to my local Trek store, ordered Trek Part #424838 for $20. To my surprise , it came in official looking Bontrager packaging:
This mount is well built and crazy adjustable. Problem is, the Garmin base has rounded edges:
I decided to dremel the rounded edges and rubber band catches off the Garmin base, flip the little set screw around in the Bontrager mount, and then use two washers to take up the gap in the mount so the Garmin base sits perfectly flat. The base has a hole right in the center as well, so the Bontrager set screw fits perfectly inside. Here's a quick mspaint:
After some work with the Dremel, the edges are gone and the Garmin base is flat. This picture also shows the reversed set screw in the Bontrager mount:
Drop some superglue on the set screw, add the two washers. More superglue on the mount where it meets the modified Garmin base, and drop on the base. Notice that the set screw is recessed nicely into the Garmin mount so the Edge 500 can snap on without any issues. Also, the tabs on either side of the base that secure the unit are free to flex and hold the Edge 500 in place as designed by Garmin.
Here's the finished product. The mount is completely adjustable; you can move it as high or as low as you want. There's a bit of a space between the unit and the bar, so I assume it would fit a Edge 800 as well.
Simple, adjustable, and works great. I like that some vendors are stepping up here, but I haven't seen something that beats DIY just yet.