The part has now been repaired. My solution consisted of gluing a large cork into the cavity (plenty of glue surface to work with), drilling and counter-boring the cork to accept the mounting stud and nut, and placing a washer in the counter-bore to tighten the nut against. This solution works perfectly since the nut only needs to be tightened until the cowl is snugged down and held in place. The repair is totally hidden when the small plastic cap is in place.
Any takers on this now-repaired part?