I've just been lurking lately but following your progress with great interest.

I'm guessing you are going to make a female mold from your foam plug. In that case I'd lay a .5 or .75 oz layer down first over the foam. Lightly sand this and add progressively heavier cloth to make a strong layup. Then I'd finish sand and follow up with a .5 layer and finish the skin as you want it. I would guess that you will need 3-4 sections in the mold to get a good release. Plan where these parting lines are going to be and how you will operate them.

From here it will be a matter of making sure you get a good release from the to be built in place mold. Essentially you will build the mold right on the plug. You probably will need some mat and heavy cloth for strength. You probably will want a gell coat first to gather the detail and finish. You will need some kind of frame work to ridgedly hold the mold together with some built in alignment holes and pins.

I'd leave off the exhaust detail and let your customer do this with your carefully detailed instructions. You would only need one mold this way.
Since you will be making 4 cowls per plane this mold will get a lot of use so build it strong. I don't think you could over kill it.