Andy,

The only one I've seen that wasn't a molded type was made from sheet balsa. Cut the cylinder shape from 1/16 balsa sheet and use as a back plate for the fins. Cut various sized "half round" fins from the same balsa sheet. Glue the fins 90 degrees to the cylinder form plate. Space the fins about 1/8 inch apart. Use any plastic "bowl shape" cut from a plastic food container for the crankcase portion and glue this to a balsa plate. Then glue the cylinders to the crankcase plate and paint. Add fake plugs from pieces of white nyrod. Add plug wires and aluminum tubes or painted dowels as push rod sleeves and your done.

Good luck,
Paul