With it heating back up again I've decided to stop letting the air get wasted when I have the top down. I've searched but I haven't found anything about a cover that will allow you to close the small air vent on top of the dash. I suppose I could use tape but I was hoping someone might have a better idea.

 

Set the a/c to the floor vents and put it on recirculated. With the system off, any blow through is put through the floor. It seems to work for me.