I picked up a TV wall-mount bracket from Menard’s for like $6.

The bracket has a handy built-in bubble level and I attached it to a 2x4. When I attached it to the sides of the cabinet, I only used one screw on each side (at the top of the 2x4) at first. This allowed me to hang the monitor on the bracket & pivot the 2x4 until I was happy with the angle that the monitor was sitting at. Then I added one more screw (at the bottom of the 2x4) to each side.

Here in its final resting place.