i see what you're saying, but i was only suggesting because most aftermarket intakes are made of aluminum. all of your choices are good, but they have their downsides. aluminum/steel are probably the most used, is easy to come by, and easy to work with. the only problem i can see with aluminum is that its more difficult to find someone that can weld it rather than steel, which isn't difficult to find and do. and like you said before, they conduct the most heat. silicone/rubber intakes have never appealed to me. i know on turbo cars, such as SR's, they have rubber intakes from the factory. the only problem with that is while under boost, the intake collapses under pressure, but you shouldn't have that problem with an N/A motor, so it could be worth a shot. as far as plastic goes, i think it may be your best bet. its cheap, easy to work with, doesn't conduct as much heat as metal, and you can always paint it to make it look nice. the only problem with this is if it gets too hot, i would worry about it melting.
i should've really thought about it before i suggested aluminum, because now i think the silicone or plastic would work better. but thats just my 2 cents.