I have one of these on my 6.3m Northcraft:
Not a bad fit around the bow and stern area but it could do with being wider where the console is (my console is tall with a high screen). Quality is very acceptable for the price. I have also seen the covers mentioned by Andy above, which are also very good for the price (same issues with fitting on my RIB though because of the console height).
Personally I prefer silver covers because they keep the tubes cooler in the sun. There are a couple of different makes on eBay.