The author builds a jig that bridges across the top of a frame for a gable-end vent. Shelf pins in the jig register in the holes on the pegboard work surface.
The top of the jig is made from two layers of beveled siding that position a plunge router at an angle.
By running the router along a slot in the jig, he cuts angled mortises across all three members of the frame.
After routing the mortises, the author ripped louvers so they are flush to the back, providing a flat surface to which bug screen can be attached.
A steeper mortise angle and wider louvers would provide more weather protection, but in this case the finished vent would be installed under a wide rake overhang, which offered enough protection from the elements.