If the cowl is ABS, there is a good chance the epoxy will not adhere permanently.

What I do with ABS is:

Thoroughly clean the cowl with soap and water. Then go over the inside with some alcohol and a CLEAN RAG.

Cut some 2oz - 6oz fiberglass cloth to cover the hole and extend about 1 1/2 - 2" beyond the hole. In your case you will need a piece about 5" across. Lightly scuff the inside of the cowl in the patch area with some 100 - 150 grit paper. Clean the residue off with some alcohol and a clean rag.

Adhere the patch to the inside of the cowl with THIN CA. I use a piece of 1/8" music wire to push the fiberglass into the little grooves and gullies.

After the patch is glued in place and dry, fill the outside with spot putty (red stuff, comes in a tube, can be found at Wal Mart, Home Depot, Lowes or an auto supply house ).

When dry, sand, prime and paint.

For about $5.00 in materials it may be worth trying before spending the money on a new cowl.