I'm assuming it is vacuum formed and not a fiber glass cowl?

These are typically a ABS or polystyrene type plastic

You can use material like this
http://www3.towerhobbies.com/cgi-bin...&I=LXDL95&P=ML
To make a joiner on the underside. You can use screws on one side and glue the other side with pipe cement

If in fact the cowl is vacuum formed you need to reinforce the back side with fiberglass cloth using pipe cement from the hardware store

http://www3.towerhobbies.com/cgi-bin...heet&search=Go

Epoxy will not stick to plastic