At least one of the ROP suppliers must have patterns for Herald Coupe as there are a couple about. You could try Canley Classics for info, Dave there has an original rallied Coupe. Though it Maynard date from before the ages of cages.
As a cage has to be made to approved designs as laid out in the Blue Book I am surprised the one you have contacted are asking for layout details,
John Davies will no doubt be along soon, he has a cage in his Vitesse, and will I am sure let you know where it was made.
Colin