Ducksback Cover - Excellent
My new to me boat did not come with a cover so I spent £57 on a Ducksback one from ebay. I was not expecting much but when it arrived I am very very pleased with it. As advised I bought the smaller one based on the size of my rib after measuring and it fits perfect. It is sort of a stretchy material and breathable. It is quick to put on and off and fits my boat so that water will not pond.
In short if you have no cover try one of these. For the price why wouldn't you.
Time will tell how long it will last. If I get more than 2 seasons I will be happy enough. 3 would make it excellent value