sub mount location

I would like to modify the kick panel in front of the drivers seat to mount my sub. There is plenty of room to add a box behind it but is it really necessary? Will the area behind the sub suffice as an enclosure? and if I do it this way will I need a sub suitable for free air use?
2002 Launch SS

If you are not going to use a box, then yes definately try to use a free air sub. Subs with a enclosure (box) tend to sound better be louder and more accurate.