The determining factor for the location of cradle length will be based on how far apart the pads are. And pad position will be dictated by where the hull is best supported (in most cases, where the bulkheads are located). Simply picking a cradle length based on a random sampling of boats in the marina is arbitrary.
I agree that contacting the manufacturer is the best course.
If I needed a cradle, I suppose I would consider building one, but only after finding out what it would cost to have one fabricated out of steel, with adjustability for the pads.