Actually, if you do the huge square method, you'll wind up with some nice HOMs in the sky for some weird reason. My best advice is to make the barrier hug the outskirts of the level by around 64 units minimum (although I'm uncertain as to how thin it can get before you can thok through); that way, it'll eliminate those HOMs, often keep a one-sided linedef nearby at all times to cull the rendering, and generally make the level look a lot neater.