3 inch tube steel frames 20 feet long and 5 foot tall with sheep panel welding to them are kind of hard to beat. Pin them together. Mine have a top, bottom and mid rail. Verticals every 10 feet. You can configure it just about anyway you want to. Big circles, alley pens etc. Then can be anchored with T posts on the outside if need be. Move them around, clean up the area, move them here to the house etc.
You'll never be stuck with one configuration. You can set up anywhere and make it as big or as small as need be. Or you can just set them up and leave them be. Nothing is ever broken. Never a protrucing nail. Narrow alleys for calves only etc.
They'll never burn down if your neighor sets the whole country side on fire. If you get burned out one time and see years of work go up in smoke, you'll never feel the same about wood again.