Following Sam's plan, you can shape the Plexiglas to a compound curve, matching that of your cowl cutout by using a heat gun and gentle pressure with your hands (where leather gloves). Once you have it shaped, you can trim it to the shape of the hole. You may want to lay up a lip around the hole, if you want the plex to bond flush to the cowl, or just bond the plex to the inside of the cowling, in front of the light.

Cheers,
Jay