How about a ball cap with 2 water bottles attached to straws, lol.
Creativity AND practicality seems to be tough in this situation. Is the horse pasture boarded or does he have a stall at night?
If he has a stall, then someone is there 2x a day to feed. He can be offered water then.
Or have the bucket mounted in such a way that it cannot be knocked around (top and bottom mounted), and high enough that it is hard to splash much out. Then put a ton of straw underneath so even if he splashes he won't be standing in water.
Or tie his tail to a crane and lower him to the water.
Other than that, I am tapped.