absolutely do not put metal on the firewall. Especially if you are using 2.4 ghz. If the plane is flying straight at you, and your signal is trying to go through the firewall to the receiver, you will have a huge shadow, and may lose signal. All of the metal on the engine is bad enough, but there are still exposed "holes" for your signal to travel through. Now if you cover the firewall with a metal foil, you might as well forget it if you ever point the nose of the plane directly at you. Not necessary and asking for trouble. I have several gassers, as do my dad and brother. All you need to worry about is isolating the power (which you have...the fiber optic is the best way to go) and isolating the servo from the engine with a nylon pushrod...as has already been stated.