The opening between return air and blower box is about 16x22 (the
filter that was jammed in there is a 16x25)

There's really no room to create a larger opening, unless I go w/ the
tipping idea, but that's tricky also (really no room to tip)

Now thinking of creating a "slot" for the filter by somehow installing
a vertical "frame" just inside the blower box which will support the
filter btwn the blower box and the incoming return. I'll need to cut
away a tab of the blower box, and and cut a frame out of tin and
fasten somehow. It can work, but I'm still w/ a 16x25 filter.. and
the idea of more surface area would be great.

With no filter, I get very noticeable AIR from the vents throughout
the house. With the filter, it's not nearly as noticeable.