We have had a 16 x 20 one of these that is going on its 4th season.

http://www.tarps.com/classichp.htm

You have to buy the chain link fence top rail and cut it to size but they go up quick, are very sturdy and so far durable as well. Make some weights out of a plastic buckets and Quik-Crete or use a couple of tires to hang in the outer corners and strap the inner corners to your trailer.

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Tom Sprecher
Team SuperTech