I have used trampoline fabric for rear coverings in the past with great results. It is easy to clean, is extremely strong, almost rip proof,, wicks away most elements, and comes in many colors. The problem is that it does not like to hold shape as well as sail cloth and is far more expensive. That being said I have been looking.
There is another way to make the roof and that is, like a Carson top. Make the body and door posts by reshaping a steel body. Leave the top of the roof open. Form the roof contours out of strips of 1/16" or 1/8" thick steel, 2" wide. Bend the strips to the contour you want starting with one that runs from front to back in the middle of the roof. Add more strips about a foot apart. Add a few more from side to side. You will have a very strong framework. Cover with wire mesh or chicken wire, cotton padding and cloth. Then cover the whole thing with a vinyl top or sailcloth top. Carson tops were made this way using strips of steel, angle iron, and tubing to make a removable top for a convertible.
I looked up Weymann-style framing joints. I've never heard of that type of construction. I found this: http://www.coachbuilt.com/bui/w/weymann/weymann.htm Very interesting!
Yes, read over that and after reading I seen just how I could use that method of the joints. But then my mind went to safety. The original idea was to put a steel top onto a aluminum substructure and use the frame I shown as a structure for the body as well as a cage. After the recommendation on searching for cloth bodies I came across that link a few post ago. Since then I have decided to lay the fabric over the chassis and use a series of aluminum bows, cables, webbing, and fabric tension to hold the shape. I did not mention that when I showed the little model. But, the paper represents cloth not metal. I will have a aluminum floor, steel engine enclosure, cover for rotating parts, and a windshield. Everything else will be fabric. The 5 plastic windows will be attached with zippers to allow easy replacement or to go to a panel look if so desired. It should provide a very light body/frame. I am confident that I can design/lay the fabric down in a manner to provide cabin quietness that is close to most newer soft top convertibles. I may ruin a few yards of material getting there but I will get there. Thanks for all the help.
The model looks like the old Superleggera (super light) or Birdcage sports cars built in Italy from the late 30s to 1960. They had a light tubular structure with aluminum panels, they had felt or cloth between the steel cage and aluminum to prevent squeaks, wear and corrosion. Did you notice the original Blue Train Bentley was a Weymann fabric body? The Airflows have a fabric insert in the roof too. The Weymann system was much more than a conventional wood body frame with flexible joints. It was designed on different lines altogether. As time went on Weymann developed ever lighter, stronger construction methods. Like drilling out thick wood beams and driving a thin wall steel tube inside, for lightness and strength. Or making a 1" thick floor out of an egg crate grid structure with thin plywood on each side. I like the idea of a fabric body with steel framework very much. It would be light, strong, and easy to build. You will need wire mesh and padding to fill out the contours and give it a smooth shape.