I put some wood backing to give the screws some bite (applies to canopy and bellypan too). They'll vibrate out of the balsa pretty quickly. Even with the backing, I have had a couple vibrate out so I am going to tap the hole and use a nylon machine screw instead of the metal self tapping screws.

I have attached a picture of my cowl showing the engine cutout for a Saito-100. The muffler was a beast to get around.