You can mount a box on the shelf facing the front you just won't have a very clear view through the back glass. I have seen boxes built that hinge where the flap that covers the tops so you still have a place to put the tops. I always put mine in the back seats and buckle them in, the tops not the speakers.

 

i only have a single 12 rockford fosgate and a 500 watt amp that kicks but and only weighs 20lbs if that including the amp

 

In my old 95 z I had the amp on that shelf and I had a box over it with 2 10's. it wasn't bad and really didnt block the view out of the back window either.

 

you check these out for some ideas http://www.subthump.com/acamarottop.htm

 

i put an eight incher in that useless part behind the left rear wheel. click the link to see a pic or two of it. http://www.cardomain.com/ride/852895/7

Garry