I built a cradle for my Hurley 22. It was a touch oversize and when I swopped for a Hurley 30 the boat fitted. The full description was published in PBO sometime in the early 90's. If you want I could find a photocopy....
No exact sizes I'm afraid, but if you PM me, I can send you pictures of the cradle I have used for the last seven years for my Rival 34. A stout six-legged affair that has withstood the heaviest storms without any problems. Heavy galvanised steel.