Try Brookes Tarpaulins at Penmenmaur. They did a cover for me. Came to Caernarfon and measured the boat on trailer, went back made it then came back and fitted. They also repaired it last year where mice go at it in a garage! It was for a 4.8m rib and cost 300 3 years ago
I am looking to get a new one for the RC585 in the next couple of months and will use them again.