We made the plug for our solarcar out of insulating foam. DOW donated it so I don't know about the cost. We used a computer to print out 80 ( yes eighty!) cross sections affixed them to the foam and carved them out. ( follow the link and you'll see it being done, in a campus bar no less ). Each cross section had centering holes that we ran centering rods through. The sections were then smoothed out. After the plug was done we covered it with the composites ( kevlar, carbon fiber, nomex, carbon fiber and a final layer of glass ). Then vacuum bagged it and autoclaved it.

good luck