Making the board portion of the setup is the easy part. Making the case, so it doesn't leak, will be the real test. Since you have the 'glass version, you should strongly consider a 'glass case, not a bolt on aluminum sort of thing. If well designed and laminated, you'll have a continuous bond, that can't leak. Also find out what the ballast is, even if it simply means weighing what you cut off. You can still have a plate board, but it would be best to have ballast attached to it's lower leading edge.
Assuming what would be typical for a boat of the original's arrangement and era, you'll want about 1,000 pounds (453 kg) of plate with ballast attached. Leaving this off wouldn't be a wise thing to do. She'd likely get knocked down pretty easily. This assumes about 30% in ballast ratio, which would be typical of a boat shaped like that and of it's era.
Simply put, a hunk of plate just will not do. For example, lets say you employ a 6' (1.8 m) long by 18" (.45 m) tall plate (some stays inside the boat) for your centerboard (about right on that size boat). This is about 92 pounds (42 kg) of 1/4" (6 mm) plate (depends on what type of steel). This is less than a 1/10th of what you need. Even if you use 1/2" (13 mm) plate, you'll still need over 800 pounds (370 kg) of additional weight to get her to float on her lines and not flop over with a modest gust.
Making friends over at the class association and get the actual ballast figures, would be the logical route. Lastly, the board will need to pivot and a ballasted boat will need a really stout pivot. This is easier said than done. You might consider a stub keel, to house a portion of both the board and the ballast. This relieves the pivot pin of some of it's burden and allows you to carry a thinner/lighter board.
