fiberglass to fiberglass strength?

Gotta question for you guys.  I've made plenty of stuff out of FG, but nothing this big.  Here's the deal.  We've got a pre-production Eclipse we're building for Pepsi/SEMA.  We're gonna ditch the rear seat and build a FG box for 4 inverted 10's.  If we remove the seat, put down some plastic sheet and lay down FG for the base, how do we go about securing the next fiberglass layers for the top to the base we already made in the car?  Kinda hard to explain...  I've got no problem making the fiberglass bottom, then sitting up the rings and stretching fleece for the top, but will the fiberglass for the top bond to the already dried bottom ok?  4 tens and 3200 watts will creat a lot of pressure!  It's been my experience that glassing to an already dried piece of FG does not create a very strong joint/bond.  should I try to find a way to add a strip of MDF between where the top and bottom bond?  Last thing I need is for someone to blow the top off this box in the middle of the SEMA show... 

Thanks in advance for any help!