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).