Instructables

Step 6: Keyboard platform and desk attached to treadmill

Picture of Keyboard platform and desk attached to treadmill
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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.