No two are exactly the same - tho another one of the same vintage should be almost right... AND the curve on each side may actually be different.
If you can find some cardboard sheets - preferably double thick you could make a pattern. Cut it into 3 pieces paralleling the frame rails - the center will probably be a rectangle or near rectangle - just jam it all the way into the C channel to get the measurement right, mark where it hits the frame rails or cross members on each side and remove. Then start whittling away at a guesstimage curve on each side and fit/refit/refit/refit... you get the idea. Once you get both sides done put all three pieces in to check the fit. the thicker the cardboard the more accurate you'll likely be, if you also remember that you may have to BEVEL the cuts on the plywood.
If you cut away too much of a curved piece, you can take posterboard (or old shoe box cardboard) and recut that portion of the curve and test fit it until it's perfect, then just tape the piece down to the cardboard.
(I helped one guy do this 10 years ago... hope my memory hasn't faded all that much. Do watch out about the thickness of the plywood you choose. Some years used 5/8 and some used 3/4... or maybe 7/16. I understand a few people have chosen a thicker plywood, and have then routed the edge down to fit the C channel.
Personally I'd rather make upholstery patterns.
Today is a gift, that's why they call it the present.