@Jim Gilbert - Madison, Mississippi Was the starting point for your project one of those crescent-moon-shaped things, or a true half-tonneau like the one on the silver speedster in the Rare Air topic photos (https://flic.kr/s/aHsmVUvSDS ) . I've never seen one of those for sale, but it was not a solution for my cover/seatback dimensions problem anyway.Other points of interest from discussion with the car interiors guy: Stayfast full tonneau about $100 more than vinyl just for the material. Sonnendeck and other German canvas completely absurd in price. Canvases are somewhat limited in color selection, especially in white, where the closest is sort of an oatmeal grey. I ordered vinyl, for price, for the bright white, and for the ease of keeping vinyl clean.
My car had two tonneau's. I cut the half tonneau. What I wanted was a top boot like the convertibles have. The half tonneau catches wind. It flaps and pulls at the corner fasteners; the ones with the turn latch near the rear of the doors.
Hartz canvas is similar to Stayfast, Canvas is the conventional material used on European and classic car tops and covers. It is an indication of quality like leather. The extra $100 is worth paying.