having failed to source a direct replacement, i have found these. My rib is 5.5m Loa, 2.28m Boa. This equates to 18ft long, 7.4ft (or 88 inches wide).
I emailed the seller and he reckons "they arent really suitable for ribs as they arent wide enough to go round the tubes"
As I can't find anything else i am tempted to risk one of them. Presumeably the biggest difference between a rib and the speedboats is the height of the consoles and seats which uses up some of the width of the cover.
What do we think?