The barn builder mounted big basket fans up high on the inside corners of the stalls, near the apex of the roof. The fans move a ton of air and definitely make the stalls much more tolerable, but I have noticed that the air they moved was typically about as warm as what was in the stalls already, if not warmer. Well, duh, because hot air rises! I wondered if this might be interfering with the action of the ridge vent.
Noticing that the aisle is always so much breezier and cooler, I decided to try mounting a fan horizontally on the stall front instead, hoping to pull cool air in from the aisle to the stalls. Rather than the white $20 box fan you typically see in barns, I chose an all-metal one with a sealed motor, for safety reasons. Lo and behold, the air in the stall with the front-mounted fan now feels 5-10 degrees cooler than the air blown by the corner-mounted fan in the other stall! The only downside so far is that the horizontal fan doesn't move the air at ground level, which is good for deterring flies from nibbling on the horses' legs. I am still definitely going to be taking the fans down from the corners, where they collect a ton of dust and are very hard to clean due to the height and their weight.
Another thing I'm considering is metal stall guards so that air movement from the aisle isn't blocked by the wooden lower half of the sliding doors. The only problem is that I will need one tall enough to keep in the giant and still growing 3-year-old as well as his mini-donkey friend who has access to his stall. I'm also not quite sure I would trust the 3-year-old, who does like to scratch his enormous bum on the sliding door. But the older horse is a definitely candidate for a stall guard and he has anhidrosis so I think he would appreciate any extra breeze!