OK maybe it's not a secret, but it was news to me. i headed into the roadster restyling project without a clue (is there another way), and after stiffening the chassis as mentioned elsewhere i set upon removing unnecessary frills (roof, doors, etc). the roof had three or four stamped ribs for stiffness and for hanging the fiberglas headliner, which went into the growing pile of unwanted steel.
i don't recall how i stumbled upon this, but when i started to lay out/draw out how on earth i was going to shape the cockpit rear, i stumbled on this particular rib and discovered the secrets it contained -- this rib has a compound curve (in two planes) that looks like a draftsman's French Curve, which it certainly would have been designed with at that time. it's a simple large radius curve, that tucks into decreasing radius towards each end. and this simple curve defines literally every horizontal curve on the car. the vertical slab sides have their own curve, very different from this one. but every single other curved surface matches this one shape. i think it's impossible that this is an coincidence.
here's the rib i use as the Golden Rule, painted a luscious yellow color to remind me not to toss it.
i think this rib was the rearmost one, back near the C pillar (above the backlight), but here i'm holding it up near the windshield for reference. the tips point forward, i think matching the rear of the inner roof. (it's hard to hold and photograph at the same time).
and here is the rib, held up to various body panels. the shape of the car front to rear is like a series of parenthesis
front ((( ))) rear
OPEN PAREN: hood leading edge, hood trailing edge, windshield; CLOSE PAREN: base of backlight on sedan (rear of cockpit on roadster), leading edge of trunk lid, trailing edge of trunklid/bumper. note closely, the Golden Rule doesn't just defgine the shapes in the horizontal plane, but in the vertical too, eg. the left to right hood/trunk/roof curve.
in this marked up photo the yellow lines show where the Golden Rule matches the panel, in two planes.
up close: "open parenthesis" leading curves up front.
"closed parenthesis" trailing curves. the roadster deck lid is a chopped down hood; the fixed portion right behind the seats ("cockpit") and the fixed section to which the trunk hinges mount i fabricated from flat stock, using the Golden Rule to shape it. both are sheet box structures, in keeping with the unibody design, and are both stiff enough to stand on. they're each made from two "Z" shaped sections welded together.
the trunk lid is stock.