The MC28 fin-keel is specified to be made in cast iron. This material is immensely durable, very strong and the piece is simple to be installed.There is no limitation of shape and the model to be delivered to the foundry is cheap and easy to build.
The building manual includes a whole chapter about the fin-keel installation, with the purpose of explaining how to build the model, and then, how to fix it. The keel bolts are attached directly on the cast iron upper flat surface which is bored and threaded for this purpose. Normally the foundries provide this service, but if not, it is within the scope of the amateur builder to do this service using a heavy duty controllable speed handheld driller for that matter. The threads are easily opened by hand.
In the United States and some countries in Europe there is some difficulty in finding a foundry willing to accept the order to produce a cast iron fin-keel. They consider the service not profitable enough to produce a batch exclusively for the fin, while being not small enough to be made with leftovers from larger services. To assist our clients in these countries, we developed another fin-keel to be made with a steel shell with lead poured inside. This keel is equivalent in performance, since it maintains the same weight, same center of gravity and same hydrofoil, perhaps causing less drag since due to the higher density of lead, it dispenses the bulb at the tip.