Thanks, guys! We've been banging our heads against a wall today, trying to figure it out. The U-posts are the heaviest they have at Home Depot, but they still bend under pressure. I like the swingset idea! I'm going to get DH to read your suggestions and see if he can puzzle it out. This pen is 12 x 13, so the smallest we have, but for a dozen or so bantams, I think it's adequate, don't you?
ETA: DH just read your suggestions and his gears are turning--thanks so much! This is the only pen that I think must be covered. Those little ones can fly pretty high and they're hawk bait because of their size.