You can use one of the shutters. The boars can't go through them. The piggies can but won't (you can push them). The piggies don't eat until they are full grown boars so they don't path. They will wander around a bit but they have no objectives (except to escape from attacking mobs).

The boars are large enough that they can clip through the walls. The piggies are small enough that they don't clip through walls.

You can clearly see 2 boars clipping through the walls in this screen shot.

https://steamcommunity.com/sharedfiles/filedetails/?id=2928180216

If you exit render distance the boars can pass through the walls and escape your enclosure.

https://steamcommunity.com/sharedfiles/filedetails/?id=2928180097