At our new farm, there is no shade where the water troughs are and the water gets HOT in blazing sun. 

I was thinking about grading a small pad for them with some M10 on it, and then having a roof built over them - like a gazebo kind of. 

Has anybody done this? Did it help keep the water any cooler?

How big would you make the pads?

I haven't done it, as I board, but I think it would help. This summer, here in South Georgia, I took the temp laser out and tested various water troughs at 5p.m. They average between 95-105 degrees. This is with them all having auto fills on them also and most being 100 galons. It is just HOT here and all of them are in the sun. I started dumping my pasture trough each afternoon after work and letting the refill fill it up fresh so they'd have cooler water. Nobody wants to drink 95+ degree water!

I moved ours inside the three-sided sheds last winter to see if less ice would form on them.  Worked rather well and so I just left one inside and put one back outside this summer.  The one inside has cooler and cleaner water as the mosquitoes don't seem as interested in the tank in the shed.  These are oval tanks so they fit up against the wall well.

I would use shade cloth it's much cooler than metal tin. It's proven in cattle research to help. I would think it would be the same for horses.