These articles represent the state of the art of 50 years ago or more. The modern expert on model sails, and a good source if you want to have a suit made, is Rod Carr.
Modern panel joint tapers produce a vertical camber distribution that is the opposite of what the 1940 texts say. I do believe that it was likely that the flat headed vintage sails were the result of two requirements:
a) As the cloth was asked to carry more load near the head, it would deform more, hence cutting hollow in the luff would remove some of that unwanted camber.
b) The camber was defined as unwanted, because camber high in the sail can result in excessive heel, and as the boat heeled in a puff, it seems logical that the vane skipper would opt for the sail to be flatter up there to release the wind and reduce the heel. We all know that balance changes as a function of heel angle, and without a rudder to counteract things in real time like we R/C types, an automatic means of controlling heeling episodes with a flat section in the top of the sail seems sensible.
The concern with heeling is reinforced by the fact that freesailing models, because they had to be able to sail anywhere in a pond, have substantially less draft than modern R/C boats, which can be steered to the deeper parts.
If you need a set of display sails made, the best approach is to locate a quilter. Quilting clubs can be found by going to your local fabric supply store such as JoAnne or Hobby Lobby. They generally have bulletin boards giving club contact information.