I would go for something like this:

https://www.amazon.com/LURIVA-Playpen-Guinea-Outdoor-Portable/dp/B0BB2NYNGR/ref=sr_1_32?crid=4W520WCALJ93&keywords=puppy+pen&qid=1679844292&sprefix=puppy+pen%2Caps%2C175&sr=8-32

I would definitely not leave a ferret free to roam outside without something blocking the sky tho. A hawk would see the ferret as easy prey, and swoop down.