Model Aircraft Wheels

[CC9]

I would like to make some large model aircraft tyres - 7 3/4" in dia - (I cant buy them off the shelf). I can make a mold for the tyre, but was wondering if anyone out there knew what the best injectable foam/rubber would be. I need it to be extremely light, but firm. The smaller wheels I already have seem to be made from a firm aerated foam, black in colour with a smooth surface (not unlike rubber) but when cut it, I can see internally that the material is grainy/aerated. Anyone got any suggestions as to what it might be? Is there a material I could mix up and inject into my mold? It needs to be flexible, light but firm...the model will weigh about 40 lbs. Thanks.