Start gluing the pieces on the back one at a time, being careful for glue squeeze out in the sliding track and making sure that the center piece (that covers the cards and pegs) can slide easily, but not to easily (I used a magnets to ensure that it stays put).
I then took the middle piece and cut it at 15 degrees as well so that I now have 4 pieces for the back. I then cut a piece of 1/4' ply for the back, I set my table saw blade at about 15 degrees and (while the 1/4' piece is still square) cut the middle section out (wide enough to cover the card and peg holes and then some) of the piece so the middle piece (that slides) has sides like / \.
What I did was router out the section for the pegs and cards on the main board, clean it up.
