I would use a solid piece that goes up one side, over the roof, and then down the other side. If you cut it properly, you can either run fine wire across the sides and roof or use screws/washers to hold it in place. It will look "finished".
IF you have concerns about venting, you can always simply cut out the vent/windows and attach the material to the outside or inside of the vents/windows. My vents open easily with the solar blanket material draped over them, and I am using the heaviest material available, 16 mil. Besides, if the weather is moderate and the doors can be left open, the fans do a good job of cooling. Extreme heat in the greenhouse is only a problem in the spring/summer though. Of course, by April, all my plants begin their journey outside. By the time the GH's get really hot, I will only have extreme-heat tolerant plants left in them, such as aloe vera, jade, and my rooting plumeria. I really don't worry too much about heat though, so long as the fans move a great deal of air during the daylight hours.
