Unused circuit breaker and switch holes are filled with plastic plugs. You'd think they would be easy to extract but I struggled with them until I found the perfect tool.

I found that the cap of a Sharpie fine point permanent marker is exactly the right size to compress the retention lugs of the hole plugs. Simply slip the marker cap over the back of the plug and pop the plug out without damaging it or the panel.

Any sleeve of the correct size would work just as well.