Four fence posts about 2' from the tree, and stock fence around, is what English Heritage used on their ground that we farmed on Hadrian's Wall. Effective, not too intrusive into the ground or the view.
You do get grass and weeds inside the enclosure though, so then the sheep stick their heads through the fencing to eat that.
. So if you make your own, make sure you tension the fencing well : tight stock fencing usually doesn't cause many issues; it's the stuff which flaps which causes the most damage to sheep and their ears tags.