I do it the "hard way".

Sand/carve a block of foam to the shape/contour I want.

Cover it with some scrap low temp film.

Apply 2 layers of 2 oz fiberglass cloth (or more depending on cowl size ) with finishing resin.

Let dry 24 - 48 hours

Sand and fill as needed

Make an X in the open area through the film and pour in a LITTLE gasoline (do this outside )

Come back in 20 minutes and scoop out the "sludge" and film

Wash thoroughly with soap an water.

Reshape cowl to desired final shape and let dry for an additional 72 hours to harden up.

Prime and paint.

You will have a lightweight cowl that is substantially more durable than the "Pepsi" bottle and a lot more heat resistant also.