murph, a hard 90 would cause problems with the air flow... air doesn't like to do sharp 90 degree bends, if you were to put a 90 in there it would greatly reduce the CFM rating of his HVAC system... this is probably the reason for the custom part, maintain the CFM rating with non-standard parts... you will need the same overall volume inside the duct, with a non-standard shape....


I believe this is the reason for the "custom" work...