I can't see the text on the plans, so its hard to tell what is meant by "false leading edge". But it looks to me like the leading edge is built of 3 laminated pieces of rectangular stock and the same thickness of square stock is used to frame out the interior of the fin. Then half ribs are used to shape the rudder and the leading edge is sanded to shape once everything is glued into place.

If this is the case, then you can build the whole frame on the plans, including the laminated leading edge. Add the ribs to the exposed side. Then remove the fin from the plans and glue the ribs on the other side. I would probably go with white glue on the laminated leading edge (instead of CA) to make sanding easier.

Again, its hard to tell based on the pic of the plans.. it looks like a fun, challenging project though!