The strongback is simply build, after the example on the site of Björn Thomasson, using two 6m long, 15cm wide and 3,4cm thick wooden boards. These are nailed together at the ends and spread apart in the middle by a 40cm piece of wood.
This creates a nice oval and kayak shaped strongback. The strongback is placed over a couple of supports to get a good working height. I do not think it will bend because of the thickness of the wood.
Next the frames (molds) are positioned accross the strongback.