Ducksback covers are good and some of the generic ones have a bit of slack which helps if you are storing long term with the outboard on.
For our use where the cover is mainly used to make our posessions a little less obvious in the day or keep sun off the tubes for beach picnics etc the Honwave cover is the business. Bit expensive but heavy quality material and very nice tight fit. Has a velcro flap to allow it to fit perfectly around a mounted outboard too. Silvery light grey fabric reflects the sun well.
See second image in this thread..
Honwave T35-AE review