I have a solid one that cuts air movement and the horses can not see each other. My mare and gelding are very close and I DO think that not being able to see each other contributes to them being more nervous.
I'm thinking of having a welder make me a new one.
When I was looking at perhaps buying a new trailer and having it shipped to the Dominican Republic, these builders had a lot of good information. I'm NOT sure if this allowed so slap me if not. :)
The Danger of Aluminum Horse Trailer Roofs