A Stroke of Luck!
When checking against my Spitfire this morning, I was pleasantly surprised to find the Herald cover is a perfect fit.
Where the late Spitfire cover differs is that the back edge is located by a lip, so only two screws are used. Fortunately, the holes for the rearmost screws are still present in the floor, so the spring cover is a straight swap, with just an additional couple of screws and spire clips needed to hold it in place.