I had the same issue once and lined the inside with ~ 1 oz fiberglass cloth. I carefully cut/trimmed a couple pieces of the cloth so it would lay flat and not overlap too much. I then sprayed a very light coat of 3M 77 adhesive inside the cowl and layed the fiberglass in place. Next, I coated with epoxy resin - two coats if I remember correctly. Finally, I sanded and sprayed with black Lustercote (for appearance only). Came out fairly good and strong. Just be careful, if the cowl is a tight fit over the mating fuse etc. you need to keep the fiberglass real thin at the opening of the cowl as it obviously decreases the inner diameter of the cowl. Hope this helps.

Steve