@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.
My tonneau frustration has been that my full tonneau has to be partially unsnapped in order to tuck down behind the seats, because my seatback is well aft of the door edge. This also means I can't leave the passenger side of the tonneau in place when I drive, because the zipper doesn't extend far enough.
Yesterday I ordered a custom tonneau to address these issues. Please see https://www.flickr.com/gp/farsightful/3bSdA3 for the outline of my bespoke tonneau cover. The purple line is where you have to fold the tonneau cover if you leave it all snapped in. The red line is the long zipper mine will have, about 47 inches rather than 36 or 37 for typical full tonneaus. The two green lines are 11 inch zippers so that the cover can stay fully snapped in and still open far enough back to fold behind the seats. The blue line is the new line along which either or both of the two halves can fold down behind the seats.
Of course, we're so deep into the summer car season that I won't see the new cover until October, but at least a solution is coming.
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.