Chris is it a tent or cover that you need.?
If a tent, the people in Poole should have the details of the one they made for Riff Raff in 2017.
If it's a cockpit cover you need, Rain and Sun in Southampton have the measurements and have made at least three of them. They made the cover for Riff Raff. For photos of cover and tent look at the early posts in my blog RiffRaff-at-sea.
Rob J