The HVAC unit in my apartment was missing the brackets to hold the air filter in place.  I designed and printed these replacement brackets to fit a standard 1 inch filter.  

There are 2 designs.  The one pictured is the ‘screw on’ type.  I also created a 'slide on' version.   You may need to tweak the sizing of either design depending on your unit, but that should be easy enough due to the simplicity. 

Note - in the picture, the filter is crushed on the sides because the HVAC repair guys were less than gentle with it.