Filler submission to the American Mathematical Monthly
The Monthly's journal page has a section on submissions:

https://www.maa.org/press/periodicals/american-mathematical-monthly#Submission

There are no listed requirements for fillers, which probably means that there are no strict requirements, just an item the editors deem appropriate.  My first suggestion is to browse through recent issues and look at other fillers to see if yours will fit in.  

Second, the above link says:


  Questions concerning submission of papers can be addressed to the editor [name + email]


So if you have specific, focused questions you can try asking.  Alternatively, you could just submit and wait for the review/editorial process for feedback.