you can buy a new one from D&R Marine. Why re-invent the wheel?
As TRINKA says, $$$$$ is one good reason, unfortunately, not everyone lives within driving distance of D&R like we do, and the o'DAY 25 rudder is not cheap to ship. Plus, if it were me, I'd enjoy the challenge of building my own rudder, possibly making a few minor changes (like less depth to make it shallower than the keel, then add an endplate to make up for that reduction in draft, like on many Phil Bolger designs.)
The rudder that D&R sells is worth every penny that Rudy charges, but stil lit is a bit pricey!