Could someone help me work out the geometry for making a scaled wooden windmill?

I need to make a model windmill. I need to make it out of wooden board so that its sturdy. Ideally I’d like it to be octangular and it would taper up slightly (as windmills do) but I have no idea where to start in figuring out how to calculate the taper and the angles to insure that it can have flat walls that taper and meet correctly at the top. Any advice would be greatly appreciated! If I have not explained it properly imagine this, http://en.wikipedia.org/wiki/Shirley_Windmill
but with flat sides instead of round.