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The size screw is specific to your power strip. I usually grab several screws and flip the power strip over and test which one is going to be the right size.I found this YouTube video on how to mount a power strip that you might find helpful.https://www.youtube.com/watch?v=-41m4NshDI0
Every power strip I have ever owned has mounting holes on the back. That is what this part of the plans refers to. **Tip - Use a pencil and scrap paper to make a map (rubbing) of the mounting holes on your power strip. Then when placing the screws, screw them right through the paper into the desk. When you are finished placing the screws then rip the paper away.
$3 Emergency Solar Radio
Air-Powered Star Trek Style Door