I was inspired by this post to try to make my own mount for the Garmin 500. I bought a 4ft strip of 1.5 ich wide and 1/16 (I think) thick aluminium. Stuck it in a vice, heated it up with a blowtorch, bent the 90 degree into it. Cut with saw (cuts like butter). Drilled holes for stem using faceplate as a guide. Then used dremel tool to make holes for garmin O-rings. Sanded, smoothed corners with a file, and painted with black epoxy spraypaint. I didn't have flat black in my garage. The usual garmin O-rings are too long, so I bought some smaller ones and used a paper clip to get them on. I bought some slightly longer bolts (5mm extra), and about 12 washers to fit the bolts. It worked great, but I still thought I could do better.
So then I kept looking at it and thought about the thickness of the aluminum. It turns out the stuff I was using (I think 1/16th of an inch) was perfect to make a direct mount for the garmin. I went through the same process above, but this time cut a perfect 1 inch hole in the plate using a 1 inch drill bit, then used the dremel tool to make the tabs. I test fitted the garmin and it is super snug mounted directly to plate, no Garmin adapter needed! Total cost was about $10 in materials and 45 min for each plate. The plate weighs 8 grams. The garmin is really snug and I don't think it will rotate without significant force. This is on my race bike, so with normal racing use (5-10 events) a year I think it will be fine. Either option works pretty well though, but I like the minimalist 2nd prototype.
I mounted on my stem upside down to raise the Garmin a little. The washers raise the plate away from the curve of the stem and are easy to adjust to make it as close as possible without contacting. I think it is about as clean as possible now. I'll take some pictures tonight with the Garmin on it. This was a really fun project and thanks for posting all the ideas.