I use some tape to hold the cowl in the correct alignment, then add 4 or 5 SMALL DROPS of THIN CA to the inside of the cowl. After that has dried I take some 1" wide strips of 2 oz fiberglass cloth, lay them over the seams and glue them in position with thin CA. I also CA a strip of fiberglass to the inside of the outer edge as re-enforcement for the screw holes.

A tip for you..... Take the scrap pieces of the ABS cowl and cut them into small pieces (about 1/4" square). Put the pieces in a small jar with a SMALL AMOUNT of acetone and let them disolve into a thick "slurry"/paste. Use the slurry to fill the seam on the outside of the cowl, Your fill will be colored matched to the cowl, and if you work carefully, there will be almost no sanding involved