I too have build box beams out of 3/4 inch plywood and they are quite strong with a little bit of flex. One is a 2x4 for the Guillemot and the other a 2x5 for the Cape Ann. I used two canoe strongbacks, as described in Canoe Craft by Moores, built of 3/4 MDF for my base and set the building form on top of these. I am building a Guillemot 19 Expedition and a Cape Ann Expedition in my shop on these set ups. It's working out quite well. As far as using MDF for the station, I would go for good plywood or particle board. Take a waterbased glue such as Weldbond and dilute some until it is like paint. Brush this solution, at least three coats, on the form edges . This turns into a very hard plastic and helps hold the staples. On the Guillemot I had a problem with the bow and stern pieces breaking. These were made from particle board. I would use plywood for these parts in the future.
Another problem I have is that using Western Red Cedar my station were not faired and when I stapled the strip to the form I have some major fiber blowout inside the hull. This only happened on the brittle WRC and not on the Northern White Cedar. I will fair the station edges in the bow and stern area on my next prject so this will not occur again. So far the projects are coming along fine and I am starting the deck on the Cape Ann today. I have been inspired by the many ideas that this BBS has put forth. When I have the pictures I too will share with the group. Regards Erich