Professional builders always use female mold to obtain multiple copies of a certain object, this requires a smooth plug, we will not use trhis method, we will enjoy fairing our foils after having applied generous filler...
We will use a "half-male mold" method to obtain our appedages.
How to do it? It is not hard at all. Let's start from the beginnig: we have to shape a foam block into a foil like shape and then cover it with whatever we want, we could shape a rudder just in one shot and then cover it on both sides, but it is a little tricky to vacuum bag a fragile piece of foam on both sides, it would be easier to shape a half foil, one side convex and other side flat, lay up fiber only the convex side of the foam and vacuum bag it while it is standing on the flat side over a flat table, then just repeat our cheap steps steps and then just glue the two piece together, one question arise now, how to glue the two pieces together? It is easy to glue the two flat sides together but how can i ensure a tight join between the two fiberglasssed skins? Just remove a little bit of foam with a cutter on the foam's trailing and leading edge, exposing by this way the inner side of board skin, then fill the gap with light filler (epoxy and microballoons), this will make the gluing surface of the foil's skins wider, thus allowing a strong joint also between board's skins.
But how to cut the foam? simple you have to build a foam cutter, it is not a complex machine, here below it is a scheme of the whole, the main part it is the hot wire, it is a thin metal wire kept straight betwen two arms, one of these last two it is mounted over a screw that allow him to rotate over the screw axis, the wire is kept straight thanks to a turnbuckle that controls the cold tension of wire and with a spring that helps to straighten the wire while it is cutting hot and then have its elongation