Peter Breu replies: I formed a seat that much resembles a Windsor seat a few months ago using 7 layers of thick (1/20˝ each) veneer. I used a vacuum bag with a male mold on the bottom and only a foam mat on top and got very good, repeatable results. I used urea-formaldahyde glue so the final seat is very strong. It is about 5/16˝ thick and is supported on four sides by the chair frame. This however, is not at all how a Windsor seat is supported and I don’t imagine it possible to put legs directly into a seat like this.
Making a formed seat thicker than this would be very challenging as the veneers would most likely rip or wrinkle if they were much thicker. With a male and female mold and steam you might well be able to mold more layers, but that would be quite an undertaking!