My pen is super open and is cleaned daily. No smell at all. But the flies DO find enough out there to make it worth a visit, apparently.
But since I've been using this fly magnet, they bypass my pen in favor of the far stinkier bait.
Go buy a fly trap called The Fly Magnet. Buy mine on Amazon. Purchase several refill packets.
Place it outside your pen. Someplace out of the way, where you won't be able to smell it, cuz it REEKS, which attracts the flies. I put mine about 10 feet away, behind the pen, where nobody...