Stan,

I would advise you to keep the concave shape as designed.  Aside from aerodynamic considerations, you would be amazed how much weight even the lightest filler will add, then offset that with the counter weight on a shorter arm.  I needed a lot of filler just to get a uniform concave shape. 

I found West System epoxy http://www.aircraftspruce.com/search/search.php  will make lighter micro than the Jeffco structural epoxy and it is much easier to sand.  

Steve Colwell  Legacy