I have an Ergotron LX monitor arm with an over/under setup. The top monitor did not angle down far enough for my liking, so I made a quick beefy bracket to help out. If you use the STL or STEP files you'll need to make sure you add modifiers to beef up the arms.

You will need a couple of ¼" or 6mm bolts that are 3" or 76mm in length, and two nuts.