Building a serviceable rudder will set you back about two weekends. One for the rough work (making head and blade, including shaping), one for sanding and finishing.
Results will be miles better than a stock rudder.
A "perfect" rudder will require getting some experience with very accurate realization of foil shapes. However, even an approximate foil shape will beat the performance of the squashed hexagon profile of the stock rudder, and you might appreciate a wooden rudder that doesn't drag down your stern, but instead will float.
Or you can build an "indestructible" rudder - also an improvement and decidedly essential in some scenarios. Includes changes to the pivot to allow the rudder to come out of the water fully. You will find threads on both kinds of projects on the forum. The "indestructible" designs tend to use metal (at least for the rudder head).
I've
builta "serviceable" rudder. It performs well, but is not strong enough for rough usage. I've sailed it up to 25kn of wind, but in flat water and to/from a dock not a beach. Both rudder head and blade could have been reinforced with carbon strips of the proper dimension for added strength. (I didn't know about that when I designed mine - if you want to discuss it, let's do it in the proper thread as these posts here get erased in 60 days).