Fog only on/in a certain trigger/mesh?

I’m trying to add fog to the edge of my map so I don’t have to model background art. The best way I can think of this is to use fog. But I only want fog on the 4 corners of my map.

Is there any way to do this with a trigger or mesh, only allow fog to be visible inside certain meshes/triggers?

I just want to have a square around my map of total fog where the player cant see past the level boundaries BUT he CAN see inside the level boundaries clearly (No Fog).

You could use particle systems to make a deep fog-like effect. You make a fog object for each border of your map.