I think it was sealsdirect.co.uk that I was thinking of. If you look in the section on ‘Marine etc’ and go to ‘Sponge Seals’ there is an item called plain expanded neoprene. I’m pretty sure I used this for a Mk2 hatch a few years ago. They also have a skinned expanded neoprene, which perhaps looks even better, especially as it’s available by the metre. (Use their waterproof contact adhesive if you’re pessimistic about which way up your boat will spend more of its time).