Here is a project where I made a NACA 0012 foil shaped rudder for a Picnic Cat sailboat. The stock rudder for the Picnic Cat is a 1/4th inch flat aluminum plate that is 12 inches wide. Not very hydrodynamic. The NACA foil shapes are standard mathematical calculations for creating various foil shapes. The 0012 is commonly used for sailboat rudders. The thickest point is 12% of the chord length (12 inches in this case) and occurs at about 30% of the chord length.
Doing a little math, I thought that the easiest way to get a foil shape was to screed on polyester resin (aka Bondo) using a basic technique I used with plaster when building my house. I came up with the plot points, subtracting 1/2 the thickness of the aluminum plate, as I was going to use the original rudder as part of the foil. That has the benefit of a metal leading and bottom edge cast right into the rudder. A good thing to have on a boat that is often brought in to a shore covered with rocks, barnacles, and oysters.