I built a overflow on my tank 12" wide ( 300G so YMMV as to what size you do ) and about 5 or 6 inches deep on a 45 degree angle with a bulkhead right behind it ... works great and skims the surface real good
Here is a rough drawing of it ...
http://www.canreef.com/photopost/showphoto.php?photo=80
Cheers