Not sure what make ours are, but are you talking about something similar to this??
They were at the stud when I started here, I think they were imported from the US, but we recently had one made exactly the same from a local company. They did a brilliant job
The ringfort is excellent as long as it does not rain, then it needs cleaning out twice a day, the best option we find is to have it in a big barn with access to the paddock