What are your options regarding space and building materials? Considering your local I would have a good look at an open top cage, will make them feel a little more free. Just a thought.
It hasn't been much of an option for us up here in North Dakota, but I'm still wanting to get them outside for at least part of the year.
If I were to do it, I would wall in a space with marine plywood. I'd perhaps make an octagon, tall enough that they can't climb or jump out. I'd make the walls straight up and down and probably fiberglass them just to make sure they have nothing to grab onto, then put a kiddie pool in there and maybe a pile of dirt for them to dig into.
And there's another consideration, they can very likely burrow out, so you'd want to dig down a ways below the perimeter and sink your fence so they can't dig under it. Think of a tortoise enclosure, then just go a lot taller and deeper.