I added two rear beams in the cabinet for a couple reasons.  I found that the plywood walls were spreading apart the higher you went on the cabinet.  While the width was 20" at the monitor shelf, it was nearly 23-1/2" at the top.  These beams keep the walls more parallel as well as adding strength to the cabinet.

Second, the back and top panels  will be screwed into these beams.  I wanted to be able to remove the back if needed and these beams gave me something to drill into from behind.

I cut the beams at an angle.  I wanted the top, angled board to rest on this beam and be screwed in.  That would ensure the top of the cabinet would remain at 20" width.

You need to cut two 2x4s that are the same angle as the sides.  The top edge of the 2x4's height is down 1-1/4" from height of wall (1/2" inset + 3/4" ply).  I put the 2x4 up where it would sit and drew a line that followed the side's angle.  I gave myself an extra 2" to work with, made the cut, then saw if it was right.  When the cut was exact, I made the board the proper length.  You might find an easier way.  The second board was cut using the same angle.

These are screwed 1-1/4" in from the back edge.  This is for the 1/2" the front and back walls are recessed combined with the 3/4" plywood that will be covering the beams.  I clamped the top while I drove screws every 4 to 6 inches down the beam from the outside.  Once I finished with the first one, I worked on the second beam.