Step 6: Keyboard Platform and Desk Attached to Treadmill
Here's the version with the keyboard tray and desk both attached to the treadmill. Since my second monitor is so heavy, I opted to hang it from the ceiling. My laptop did slightly shake when I was walking. While it wasn't enough that I couldn't focus on the screen and work, with a wall so close, it seemed like I should take advantage of it.
If you can't attach something to a wall, I'd recommend building a desk that sits on the ground and forms an arch over the treadmill.