A suggestion for horse comfort that we use is to ice the footing in the trailer. We have a stock trailer, but on those REALLY hot summer days with high humidity here, we add ice to the sawdust bedding under the horses.
Sawdust is fairly deep, 4-6 inches, we use a couple bags of ice per horse, mixed in the sawdust which keeps it cold. On long hauls, you may need to add ice every couple hours, keep it cold under the horse hooves Friends have done this hauling to Canada in the summer, with a big load of show horses, having the normal holdups waiting to cross the Border. Horses stayed cool while parked, not tired, even in very warm temps inside a closed style trailer with the windows open. She said the horses seemed a lot more rested, not tired from the long haul using the iced bedding.
We have not had any issues with horses “slipping” on the ice with a decent amount of bedding under them to cover deep ice. Ice does melt, dampens the sawdust, not blowing around in the trailer. Or you can sprinkle the sawdust, to keep it from being dusty.
Do clean the icy bedding out of the trailer pretty soon after you are done using it. Closed trailers will add heat to damp air as ice melts, so you can QUICKLY get mold going in the trailer. One person we know didn’t clean her trailer out for a week, it was REALLY moldy in there by that time! Had to spray the inside to kill the mold when they got everything out, then rinse it down and leave open to dry.
I clean my trailer the next day after we get back home or at the show, get the wet stuff out. There is usually plenty of still frozen ice still in the bedding, so you do get a long use time of ice under the horse.
Horse will take in the cold thru his hooves, really helps keep the rest of their bodies cooled down this way. Our humidity can be very bad, so even sweating, a horse doesn’t self-cool without a breeze over him to carry off the sweat in a closed (not slat sided) trailer. Keeping him cooled with the ice around his feet is a huge help to prevent overheating on hot days.