I attached to the interior plastic. Used two bolts comming from the back side of the plastic though the box. I placed large washers out behind the plastic to displace the stress on the plastic a bit. I then ran the bolts to the inside of the box where I used a couple of lock nuts and washers to finish it off. I then caulked it with silicone house caulk to seal it up.

If I did it again I would put a bolt though the floor too because when I hit large bumps it sounds like it is shaking the box pretty bad since that sub is a lot of weight. Hard to tell but I am going to sit back there and see if it is really shaking it when my wife drives it over a bumpy road. If it is I am going to remove my sub from the box and bolt it to the floor.

As far as thickness I think I did about three. When I was done it was pretty darn strong.

Other then the bolt throught he floor issue it has worked out great. Good luck.