I have my box bungee'd in the truck. I autocross at least once a month and I am constantly pulling it out.

 

In my old car I did the same thing with a 1/4" piece of MDF mounted behind the rear seat carpet/plastic thing screwed into the X cross brace. Not sure what I'm going to do with my new car yet. If I did not auto x (have to put tires in my trunk) I would build a rear strut tower bar and mount the amps on top of the bar in some fashion.

 

makes 2 of us

 

a trick i use is buy some sticky back velcro and make 2 long strips, stick hook side to hook side, so it's the hard part on both sides. stick that between the sub box and the carpet and it won't move. that's assuming you have carpet....

 

Here you go.