I have been wishing for a temp gauge for a long time, but didn't know of a cheap, effective, bolt-on option until I started following this thread. I bought the TTO unit off ebay (thanks to Post#27 above by Seeya) for $43.90.
I mounted the sending unit in the upper radiator hose. The sending unit body is about 3/4", but I only cut about 1/2" out of the hose as the "straight" section of the hose that I mounted it in is barely long enough to accept the spigots of the unit and still fit in place. It was a tight fit!
I mounted the display under my handlebars by making a small aluminum plate that I could stick the display onto, leaving a tab above the display that I could sandwich between my Motowerk bar riser and the handlebar clamp base. (See photos).
It seems to work just fine. I took it for one test ride yesterday and it seemed to settle around 180F on the open road, and reached 198F in town (in 90F ambient).
The unit is small and light and very legible. It has a "feature" that will show you the max temp, alternating with the current temp. Unfortunately this alternating display never goes away. The unit is never off, so this morning the display was still flashing 198F alternating with the current temp. It will reset the max the next time you go above 130F, but I find this "feature" distracting and irritating. Fortunately you can turn off this feature. Unfortunately the button to set the scale (F or C) and the Max feature is on the bottom of the unit. If you use the supplied mounting tape and stick it to a flat surface, you cannot get to the "settings" button. I drilled a hole in my mounting plate so that I could access the button. I turned off the Max feature and am likely to never need to use the settings button again, but I wanted the choice.
The wire leaves the display unit on the right side. I would have preferred that the wire exit out the back of the unit and the settings button be on the front, but, it WAS cheap.
I'm including all this setup information in case any of you are considering mounting this on your dash. It would be harder to do a neat wiring job with the wire coming out of the side, not to mention getting to the settings button.
All in all, I am pleased with the price, performance, appearance and, hopefully, the long-term reliability.