My criteria were as follows:
1. Needs to be small / compact
2. Speed needs to be the primary focus (Speed is obvious at a glance)
3. Visible in all sorts daylight
4. Easy to mount without drilling holes
5. Have a quick attach / release so one can lock it under the seat if need be.
After trying a few GPS speedo devices, I think I've settle on the Meilan M3 Mini GPS Bike Computer!
1. As you can see, it's very small and light.
2. Speed numbers take up most of the screen along with additional numbers below that can be various things (Trip OD, Time, Altitude, etc)
3. Drove with the sun in various positions (direct, etc) numbers are quite visable
4. If you look at the link, you can see it comes with a base. That base also comes with a pair of zip ties. All I had to do was zip tie them to my mirror. No permanent damage.
5. The Speedo can easily attach and detach itself from the base that was zipped tied in #4. The only think is since the location I've attached the base to is not vertical, it doesn't technically "snap" into place like it should. However, in my driving around, this has not been an issue. The only thing I've noticed so far was when I was going through some rough gravel road work and the zip tied base started to move a bit. Perhaps some rubber sheath on the mirror arm might help but not too concerned.
The link goes in depth on all the features it has. For the size, it's a very capable device. It is battery operated and would need to be recharged from time to time. So far I've logged around 100 miles of highway and in town driving on the first charge. Those that have USB power nearby, probably could just keep it plugged in I would suppose. It also claims to have a backlight. I almost never have a need to drive at night so I have not tested this.
So there ya have it. Thought I'd share my results and hopefully someone else might find this useful.