We had the same issue with our 78 MS 1. I made a template from luan plywood. This I traced onto a piece of Starboard. Two off the shelf louvers that would "fit" the template were purchased and the center of the Starboard was cut out and the louvers surface mounted. The assembly was glued in place and recessed using Lexel white adhesive caulk behind and with a bead around the outside perimeter to stop it from coming out. Worked great for about 5 years until we sold the boat and looked decent.
It is not who is right...it is who is left.