I buy those Fourmost bulkhead fittings at my local hobby shop but Tower also stocks them
http://www2.towerhobbies.com/cgi-bin...G851&P=SM#tech .
They come two in a package, a yellow and a blue one (See picture, I've already used the blue one).
By the way, I back them up on the inner side of the cowl with a small thin (1 /32) piece of plywood. That may not be necessary but I didn't want to worry about the flexibility of the cowl.