I've just finished knocking up a sort of half-half tonneau. I was idly gazing at an offcut of Mohair and it dawned on me that I could use it to fill the hole between the hood cover and back of the seats. I didn't have the camera with me but I'll take a photo tomorrow. I think it will reduce draughts in winter. It fixes on the two catches on the rear rail, and two mohair 'straps' onto the hood frame. All will become clear.
If it's too floppy I can line it underneath with neoprene........