Here is an idea for a simple inexpensive horse pen.
Coming from the UK I find I do not have the need to keep a horse stabled overnight. I designed this pen with horse handling in mind. It can easily be used to keep a horse in overnight, and with some shade cloth would keep out a prevailing wind. My block it pretty well sheltered with shelter trees so I don't have a need to make it more sheltered.
The openness of the pen and all-around vision helps keep the most highly strung horses calm - as they can see their mates a few paddocks away, and everything else that is going on.
Because I don't keep horses in overnight I use wood chips for the floor, and try and keep it clean to make them last longer.
All the wood was new, but the roof was recycled roofing from the local demolition builders.
The dimensions are 12' x 12' floor, (slightly wider roof), door space 4', and escape hole 18", lowest roof height 8'.
I have 16.3hh horses and have been able to take all three into the pen at one time and tie them up to dry off and cover in a downpour without a problem!
I have put closing doors on and netting between the rails because I used it for a litter of puppies and more recently started some calves in it, otherwise, just a gap and a chain to close the 'door' is all you need.
The most important feature - the gap - easy access and exit! I also have a gap into my field - no gate!