+/- 5kHz deviation FM (what has been called NBFM since the 60s) is 16kHz wide, not 25kHz.

Analog FM at 6.25kHz spacing will not work;

Fin size depends on a lot of things, but typically, the fin span is less than twice the airframe diameter. Chord varies widely. Fins are typically less than 0.125in thick. Common fin materials range from plywood, to G-10 fiberglass, to carbon fiber sheets. On rockets expected to reach higher speeds, it's common to wrap the tail section of the rocket, including the fins, with fiberglass or carbon fiber cloth. While it isn't common, it is possible to use balsa for HPR fins. It just needs to be reinforced with other materials. I built fins for one rocket by sandwiching layers of balsa, paper, and sheet styrene.

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How did a question abut stopping rust turn into a dissertation about two parts of an antenna?  I guess I missed something.

Yes you did miss it.................... its called a Pro Rack