the side-view of the box is a (isosceles) trapezoid. exterior dimensions:
~16.25" tall at the front
12.5" tall at the rear
even though this box is much easier than the ported, it isn't really gonna be a breeze to make, there's a couple funny angles in there (specifically, it's about 12 degrees, i think). therefore, making the top piece will be a little tricky. the dimensions listed below are what you should cut it to, and i've left some leeway; you'll need it, because no tape measure will get you to the precise value you'll need. you'll have to stick it on top of the rest of the already-assembled pieces, and shave little slices off until it's right.
dimensions of pieces to be cut:
front: ~16.25" x 16"
bottom: 13.575" x 16"
rear: 11.75" x 16"
both sides: 12.825" on the bottom, 15.25" on the front edge, 11.75" on the back edge
top: 13.75" x 16"
to visualise how these pieces go together: start with the bottom laying on the ground. the rear piece goes on top of the bottom, at the rear (duh). the front piece rests on the ground, just in front (so when you screw it down, it's through the front into the bottom, just as the screws go through the bottom into the rear). the top piece rests on the rear piece, and just behind the front piece (same deal, you screw through the front into the top, but at a slight downwards angle). the sides can then be slotted into the frame. they're meant to be indented 0.25" each from the edge, in order to facilitate carpeting.