I purchased 2 Amazon Basic tower fans (FZ10-18TR).  My 5 year old lost the nut that secured the base to one of the fans.  I used the other as a model and created this object.  I printed it at 100% out of ABS.  You might want to bump that up to 100.5% depending on your printer.

I made it parametrically so even if similar tower fans from other companies use different threads or sizes, this file should be able to be easily modified to generate a nut for other fans (or other large plastic nuts/knobs).  The linked thread library makes this very easy to do.