About all that is inside the box is some fiberglass lining the walls and probably a single 2x4 (actually closer to 1.5" x 3.5") brace between the front and and rear baffle. I've never looked in one so I am basing this assumption on my similar JBL 4647A cabinet. A little extra bracing would be a good thing. I would say are on your own for picking a port...whatever you like. Nothing magical about 1x 8" or 3x 4" or whatever.
3/14/14 at 4:44pm