Stainless steel, or titanium if you have the budget for it, should serve as your main firewall. However, the composite structure must be protected from heat by Fiberfrax or a similar material. The Fiberfrax can be protected from oil with a thin layer of aluminum applied over it. Thus you have a sandwich of stainless steel/Fiberfrax/aluminum. The aluminum will be on the engine side and can be quite thin, say .020" It is sacrificial in case of fire, and is merely there to protect the Fiberfrax.
In testing Glasair found that they needed 1/2" of Fiberfrax to keep temperatures on the cabin side of the firewall low enough to make them comfortable. You must remember that even though the stainless will stop the flame it will not stop heat. If the temperature of the cabin side of the stainless steel firewall exceeds about 700 degrees F the composite structure can burn.
Lancair makes a preformed firewall blanket for their kits that has a bonded protective coating over the insulating material. I don't know if any other kit manufacturers do the same, but they should.
When you look at other materials besides Fiberfrax be sure that they are fire rated for 2000 degrees plus. Many of these materials are heat/sound insulators but not suitable for firewall protection. Also be sure to use caulking rated for 2000 degrees, not high temp RTV which is only good for 500 degrees continuous exposure.