My filter is on the inside of the blower box just ahead of the fan.
When the blower is running, it naturally tends to pull the filter
towards it. To hold the filter in place, two heavy guage wires wedged
into the upper left and right edge of the filter (and down against the
floor) and the bottom of the filter kinda/sortta sits in make-shift
ledge.

Seems to me a better design would be to relocate the filter outside
the blower box so it is pulled tight against the box when running.

But the area outside the opening to the box is 22" wide. The filter I
have now is 16x25, and the next smallest I see 16x20. That would mean
somehow making the opening smaller. and then what impact on airflow?

any other ideas?