Earlier quoted context omitted.
Even easier: just place the 20”x20” filter BEHIND the box fan. The vacuum will keep it in place nicely. Be sure to note the airflow direction arrows on the filter.
I put it behind the fan as well, but now I'm curious if there a difference in air quality between the two methods?
One thing to keep in mind though is most filters have an expected direction of airflow, which does matter both to its efficacy and efficiency.