Back in the days of my Ham hobby, many vertical antennas had ground "Planes" constructed out of four horizontal wires parallel to the ground - sometimes buried so you could still mow the lawn. Where we build light for short field or fair for speed, Hams build for distance. If six were better, you would have seen six or even more. You will often see this type of antenna on the roofs of fire houses and other structures that house radio transmitters - the four horizontal whiskers are the ground plane. I agree that the fuselage is a great ground plane if using the welded antenna mounting plates. With the Dorsal Fins that some are adding that extend from the vertical stabilizer to the turtle deck, the plates become unusable and a suitable ground plane needs to be installed. I terminated the dorsal fin tube to the aft edge of the plate for this reason. A ground plane can be made using appropriate lengths of adhesive backed copper tape soldered to a brass ring that mounts to the base of the antenna. I don't think you need a round metal plate. Also keep in mind - and this is from my aluminum wired house days. Aluminum will corrode next to dissimilar metals unless well protected and will eventually cause lots of buzzes and sparks.

With regard to Coax runs. My current radio install manual says separate. Whether it is a wives tale, or not I followed that recommendation. And regarding Bob Knuckolls recommendations in general. He was a participant in the first Kitfox email list years ago and made some comments that implied to me that he had opinions just as we all do. Example: He couldn't understand why anyone would want to put in expensive circuit breakers when simple cheap light weight fuses did exactly the same thing. Well, I love my Klixon Circuit breakers that allow me to simply pull the button to turn off a circuit for testing or just because sometimes without having to find and pull a fuse and remember where I put it when I need to re-energize the circuit.