You know how to make control sectors, right? Well, make one of those. Give a different tag number to the sector you want the FOF to be in. Use that number, along with linefef action 25 on the control sector's linedef to create a FOF over that area.
The ceiling and floor heights of the control sector control the top and bottom heights of the FOF.
The ceiling and floor heights of the control sector control the top and bottom heights of the FOF.