put a whirly bird in the top of the float - that will draw out some of the heat, and put some vents in the sides of the float as well.
Pop out vents just in front of their heads on the sides will also help. You can close them in cold weather
We have air conditioning in our truck (an actual in the side of the house air conditioner! lol) but its only for when we pull up at a show and can plug into 240 or run a genset and its ONLY in the people section which is insulated cos it used to be a refridgeration section in a truck! lol.......
I dont think 'air conditioning' in the horse part is a really doable thing as you would need a power source and you would need a huge powered air con to take away the body heat of one or two horses, and it would have to be totally sealed.
I would go for vents, pop up vents and whirly birds

Good luck!
