Align the halves clamping it with the flange.(3 or four places at first ) Use your finger feeling the inside to check the alignment; the halves should feel flush.
Apply thin CA to the seam from the inside, just enough to make a bond.
Remove the clamps and sand the inside of the seam.
apply epoxy and fiberglass to the seam on the inside about 3/4 inch beyond the seam.
trim off the flange and sand the halves flush, prime and paint.
If this was my model I would use the halves as a mold to make a fiberglass cowl.
it would go like this:
Align the halves clamping it with the flange.
Use your finger feeling the inside to check the alignment; the halves should feel flush.
Apply Wax or mold release, (wax is better because I know it wouldn't damage the plastic) Apply at least four layers of glass cloth to the inside of the plastic mold and give it plenty of time to set up.
Remove the clamps and shoot shop air in-between the flanges to release the mold.
sand the seam flush, prime and paint.
If you buy a boat repair kit found at home improvement stores there should enough material there to make a good cowl.
As for the wax; the wax used to seal toilets has such a low melting point that it can be applied with a brush if you heat it with a hair dryer.