I have two Chieftec beige box. On one of these, I couldn't find where the plastic part of the power switch went, so I made one with the remaining one (the yellowed one on the pics).

I didn't add the dome shape on top of it so that it will be easier to print. Don't forget to flip it upside down on you slicer though.

Also one thing, if you're missing a power button on a more recent Chieftec case, this one might fit. However, you may need to lengthen its Z size. You can either use the Customize app from Thingiverse, or you can use the original OpenSCAD file that I made available through the archive.