A little ways in to the project, we changed the design slightly to set the front of the shelves back from the front of the case a little bit - it looks much better this way.
We got our plywood and lumber from Boulter Plywood in Somerville. They were a little more expensive than other places, but considering they're only a few minutes away by car, the convenience of not having to drive on the highway with sheets of plywood on top of my car far outweighed saving twenty bucks. The folks there were quite nice, and were happy to make a few cuts for us on their nice big table saw.
We set to work cutting the plywood to size, and then routing the grooves for the shelves, vertical divider, and top. Notice, too, the notches in the side pieces at the bottom - these fit over the molding in my friend's house:
Check it out (the first picture was pretty bad, so I had to adjust the contrast so you could make out the shelves - it's still pretty bad, but you get the idea):