Or how about getting rid of the vents all together like a lot of us so it free flows down instead of up or sideways at the hot ceiling it made a big difference for me as well as putting PVC pipes up there to prop up the ceiling for more flow like others have said do a search and see how I and others have posted the mods like this
http://s1275.photobucket.com/user/to...remw6.jpg.html