I bailed on extending the desktop because the desk legs couldn’t support a desktop that extended out that far. Instead, I went with an old idea of having a monitor stand that just sat behind it.
I made it with a nightstand that I had previously made by attaching two drawers together. I stabilized it for a heavier top load by attaching a wooden frame to the bottom (that I got from a free ottoman, back when I was picking up free furniture for the parts). Then, I reinforced the two drawers’ attachment to each other by attaching some boards and the sides of the drawers to the base by attaching these large blocks (with curved surfaces).
I gave it a few pushes, and I think it’s fairly solid. It doesn’t beat just building a box, but I didn’t have to go out and buy lumber, and I used up some junk I already had this way.
(Plus, I just enjoy binding existing things together.)