Couple of sheet metal screws should do it. They should be located about 1" in on each side and be biased towards the far edge of the return. Return being that 1" flap at the bottom edge of the seat back. Look for the holes in the seat frame.
I thought maybe it was screws. I found the holes on the bottom of the seat frame. I assumed it would be something threaded. I guess I will have to find a couple of sheet metal screws big enough to work. Thanks for the clarification. I would still appreciate a pic just so I am certain I understand.