Build a Swing Keel from Scratch - ''''74 Helsen 20'''' Sloop
If the old keel only weighed a couple hundred pounds, I would probably start out with a 1/2" steel plate and have it cut to the rough shape of the centerboard. That alone would weigh a couple hundred poinds but you can build it out to create a foil shape using closed cell foam or plywood and then glass over the whole swing keel with a couple layers of glass and epoxy resin. As long as the epoxy barrier/cloth remains intact this should last a very long time.
If you wanted more weight, I would cut a series of holes in the steel near the bottom of the keel equal about a quarter of the steel area, build a simple buried mold, stand the steel on end and centered in the mold and cast the tip of the keel in lead right around the steel plate. Build out the top in closed cell foam or plywood and glass over the whole board with a couple layers of glass and epoxy resin.