chemie,
I used 12" extensions for my aileron servos. As ar as servos go, all of mine are standard HiTec 475's.

Fiberglass can be Monokoted, but I have never done an entire cowl. I know there is a method to stretching and hining the seams, but have no experience. May I suggest posing that question in another forum perhaps? Like tips and techniques? Possibly a Google search can bring some information to you.

I forgot to say that any automotive paint dealer can match Monokote. Expect to pay $40 a pint for it, though.