Rep Power: 0

Ok if you just want it to not move around and some security just screw thru the box into the factory piece. Yes its thin but if you put speed clips on the back side of the board then you wont have to worry about them ripped out. Personally I screwed thru the floor when i had a box because its way better security. You can trace a sub floor but odds are if someone wants your box bad enough they will pry up and the screws will pop thru 1/2" mdf. 2nd if you do a subfloor you wont be able to get it in the car as 1 piece because of the trunk opening. I live in NY and i drive my car all year and have had no problems with the screws going thru my floor and rusting anything and its going on 3 years now.

 
Rep Power: 380

Not only is custom2k1's post correct, but because you fasten the box to a part of the car you might have it covered under insurance incase of vandalism. Typically they are reluctant to cover it because they are so easily stolen.

 
Rep Power: 328

straps that go over the box, and fasten to the trunk floor on wither side (if you replaced the crappy cardboard stuff with 3/4 mdf, you could use 3/4 screws and not go into the metal) Also, if you replaced the cardboard stuff, you could run some screws from inside the box and then reload your sub. it's secured to the floor, and they have to unload the box to remove it. youcould also use industrial strength velcro on the bottom of the box. I could go on and on.

 
Rep Power: 360

Here is my suggestion

1. get a cheap 10 to 20 daller ebay rear strut bar. Like this:



2. get a peice of metal that will sandwich between the nut and the bar (see image above)

3. then take the other end of the metal that was sandwiched on the bar and screw it onto the box.

Like so: (red is the metal peices) the image draw dosen't have the metal sandwiched, but you should get the idea