I suppose it'd be a risky investment for any hood manufacturer: 20 hoods is about £8000, and how would they decide what is wanted? Two-seater or family? Red or beige? Original frames or later? I suspect the windscreen "pegs" and edge clips might well vary, too - especially when there'll have been various restorations or repairs in each car's history.
A hood can't be that hard to make - the very first time I used a sewing machine, when I was 15, was to make one for the C-Family I'd just bought. I admit I used red canvas, but it did fit properly!