Formula for a Body of Resolution

cliff

I am not a math whiz, but I need a formula similiar to a parabolic curve to design a shape similiar to an aircraft wing tank in 2D.
I have found two body of resolution formulas in the old NACA files, but only the rear aspect and thickness of the shape can be changed. Naturally.... I need to change the nose shape as well.
The shape I'm trying to design is for a motorcycle streamliner body, for running at the salt flats. Hence I have to be able to change both the front and rear ends of the shape to fit wheel diameters and engine width, yet still maintain the smooth air flow.
I could free hand the shape out, but in making the body mold it would be much better all the way around to have some formula results to work off of.
Any help/suggestions would be appreciated.