Surprising how often they come up on ebay.
Have you thought about making one from scratch?
Slimtim and a couple of others have made some some very good looking made-to-measure ones that have been well recorded on threads in the past.
Also saw one at a boat jumble recently.
RIBBED For extra pleasure.
Member of the ebay Blue RIB cover club.
Member of the Bombard 380 Aerotec club