I bought one similar to this one from Ebay for my Ring 18 from this guy:
(Cut and paste into Ebay's 'Search' field)
Not quite the same, as mine has a sewn-in cover for the outboard too, but very similar.
Absolutely superb quality, heavyweight, breathable and with more straps and buckles than a bondage convention it's not gonna blow off in the winter.
The seller (Ben, from memory) is a really helpful top dude. I paid for it on Wednesday, had it by Friday lunchtime.
I emailed him a pic of my boat and he came straight back with the perfect product.
Can't speak highly enough of his service and quality.