Cathy, have not built mine yet. Don't yet have the property, just planning in advance. Personally, for training's sake, I am going to use a wooden pen with posts set in the ground (probably good quality railroad ties), and 8' walls (either solid, or more likely 3 rails of structurally reinforced 2x6 boards. We will see how this pans out when the time comes.
I am not a big fan of the pre-fab metal ones as I have seen a 2 year old draft baby completely mangle a pen in a single bound. Do not want to see any ponies getting legs ripped up in aluminum railing should something go wrong.
I have said what I have said. I have not said what I have not said.