Best way to have a fog volume

My game needs a lot of medium size volumes in which the player should be able to enter. In UE3.5 we had fog volumes which were a bless.

While we are still waiting for epic to make them… I decided to try and make something I can use.

I need the fog effect to stop at set point and particle fog clips out of that box.
Do you have any ideas ? I don’t want to trigger Post processing when I enter the volume , and I want the fog to be contained .

My implementation looks good on the outside BUT

When you enter the mesh nobody’s home

This looks like something what might work BUT i don’t think it will make a GPU happy