I like to make a plywood ring and glue it around the inside opening to give you a small flange. Before you glue it you can center it on your firewall and drill your holes. I also drill two 1/16” dowel-pin holes to hold the cowl in place while you fumble with the screws. Then with a long ball allen head screwdriver you insert the screws through the engine cowl openings and open bottom section. This way nothing but a seam is visible from the outside.