also curve it a bit as you can get the air to follow a curve a bit .. look at a prius for inspiration.. and also make the center higher than the ends near the corner of the bed.. makes a curve to help air not get turbulent if it falls off the bed cover ..
![](http://us.autos1.yimg.com/img.autos.yahoo.com/izp/toyota_prius_touring_2008_exterior_rearview_640x480.jpg)