2-horse trailers... while I don't pay that much attention to them... or, really even see that many of them around here... I'd say probably a majority of them I've seen are open like that - that's how the old one we have is... I just tied a 2x8 that was a foot or so longer than the trailer was wide, across the back, above the top of the doors, with ends sticking out of the side windows. Cow and calf rode 240 miles or so with no attempts to go over. Granted, she wasn't a Brahman-crossbred, or wild-@ss Angus...just a little black Jersey mix, so it didn't take much to keep her contained.