The back of the box is (only slightly) more complex, because of the sliding door. We will create a canal for the sliding door to go through.

First, glue the two small rectangular pieces onto the top and the bottom face, inside the box. These will be used to carry the inside frame of the sliding canal.

Put the inside frame (the smaller of the two rectangle frames) inside the box on top of the two carriers that you glued-in in the previous step. BUT! Before you do, you will notice that the bottom face has a little half-circle opening which is used to let the power cord through, so place the power cord through the opening before gluing the inside frame. (Or you can split the inside frame at the bottom. The only reason why it wasn't pre cut is that if it were cut, the whole frame could warp a little during the laser cutting process).

Once the inside frame is in place, you can glue the outside frame (the bigger of the two rectangle frames) on top of the box. Leaving enough space for the sliding door to slide in and out.

Try if the door will slide smoothly, before gluing the top frame.