Aren't there specific angles that are required for monitors? Things like that.
There are generally accepted ranges of angles that people like, but manufacturers don't even agree from product to product what the specific angle should be. For instance, JBL PRX512, 612, and 712 each have different angles, and most agree that the one on the 612 is pretty crappy to the point of being almost useless. People have been known to carry 1x4s and 2x4s with them to modify it.
Find the angle you want and build a side board accordingly. Or go with the Fender tilt option Patrick mentioned, where a single bolt (with maybe a wingnut) attaches the leg to the cabinet. That way you can change the angle anytime you want