I'm looking for the most powerful window fan I can buy for our attic window. Unfortunately I'm limited on the opening of the window: 25" wide and 17-3/4" tall. Right now we have a Holmes fan in there with dual fan blades..the type you can buy at Wal-Mart. The fan direction is reversible with a switch but it just doesn't have enough power to move the amount of air we need to keep it cool up there. Does anyone have any other recommendations?