We still aren't sure, but pressure treated boards are out. That is what one of our major hold ups are. We couldn't find any plained boards that are not pressure treated. They are treated with arsnic and other poisons and if your horse decides to take up chewing wood.....oh oh......so we are not going there.
Jerry would like to build some like Lisa has pictured above. He can do it quickly and easily in his shop and it would be cost effective, airy, and no maintainence.
Our other idea is to use horrizontal oak planks that are in a channel so you can remove them or add boards to them. I don't think they can chew that hard oak.
I still have a month to decide.