Roughly speaking

Today I sanded down the rudder repair block to shape.  The curve was drawn from memory of the old shape.  The NACA foil was approximated by carrying the taper of the foil shape down horizontally.  I really doubt much lift is achieved this low in the board.  The main goal is to reduce energy robbing eddys (turbulence) from forming due to sharp edges.  That is the reason for the rounded off trailing edge and rounded of foot.
Unfortunately, I am out of Interlux two part epoxy fairing compound. I will fair and sand until very smooth. Then I will use e-glass 6 oz cloth and encapsulate with epoxy.  Next I will use the yellow cloth you see in the picture which is Kevlar and sheath the surface that might drag in shallow water. This will protect the wooden board from intrusion by water.  Finally I will fair over the entire surface blending the margins of the glass.  Kevlar cannot be sanded so it has to be faired up to the protruding margins much as I did with the centerboard on Belle.  When finished I will coat with 2-3 applications of Interlux Interprotect 3000/3001.  The final coat will be Awlgrip off white above the waterline (because I have a can of it and antifouling below the waterline.  I haven't yet decided on a color for the antifouling.  Meanwhile I await an order from Jamestown Distributors.
Update: I found the cans of Interlux two part epoxy fairing compound.

So here is the rudder, faired and with the Kevlar edge glassed in place. The hot weather had the epoxy kicking fast--so fast the pot was burning my hand.  Luckily we got it done before it set.

And the UPS man brought my cyanoacrylate glue.  I was able to mend the chip in the mast track.