I use a huge tarp from Home Depot. One with the eye holes on the side. I use about 5 ft of thin rope to to make a perm. knot on each of the eye holes and simply tie down the other side to the trailer tight. I don't have a wakeboard tower though. If it folds, this method should work or if you can somehow customize the tarp to fit around the tower without leaking it could still work.
It's a cheap solution but you did say 'generic'. I bought a gray tarp instead of blue or brown which looks a little less red neck.
