LOL - pure ally melts at 660°C but as soon as you atart to alloy it you'll see a reduction in melting point. Would need to know what alloy you've got to get the right phase diagram ;o)

Regarding the cold air moving inside the shield...... the whole point is you're trying to avoid that taking the heat back into the engine. You'd be better off with a polymer heat shield covered in tin foil; the foil would reflect and radiative heat and the polymer would slow conduction through to your "cool air chamber"