Make a cup of tea and have a read of this:
Those Ebay Covers
It's been on the boat for quite some time now, and has endured quite a few miles of towing at, erm, legal towing speeds, driving rain, high winds, several falls of deep snow, etc etc. with no maintenance.
And I have to say, it's now completely knackered.
However, bearing in mind what it's put up with since February 2008, and how much they cost, I'm about to buy another one.