like those thoughts..

the "v" idea.. mmmm.. will have to think about that

i thought about moving the filter to a different spot in the cold air
return flow. furnace is in basement, but there are two bsmt return
vents plugged in right next to the furnace (so I cannot tap in after
those since they are so low in the return air channel).

I have 2 returns upstairs and 2 downstairs. I think it would be good
if I did have a return in each of the upstairs BRs (3), so that is
something I'm looking at doing. That does kinda make the idea of
filter at each vent a challenge.

Upstairs I have one return high up the wall in our vaulted room (good
for AC) and one in the hallway. Downstairs there is one between the
furnace room and the open finished area outside the furnace. And
there is one in a BR that backs up to the furnace room.

The trick is my return air run (what is it really called?) is 23"
wide, which gives about 22.5" inside, so the filter closest is 20" as
the 24" is too big.

I have a 3" W x 1/2" H galv. metal piece underneath my blower fan.
Not sure if it is supporting the blower; though I see the blower
attachd from above also. That chunk of metal extends over to the
return air inlet, which creates a "bump" that the filter must span.
If I get rid of that, then I have a smoother 16" x 25" area that the
filter can be placed against. There's still no "frame" for it to rest
against.. as it's in the blower box area. There's about 3" between
the return inlet and the fan housing. I might be able to somehow
fabricate a chunk of sheeting and screw it into place to create a
filter "slot".

For returns.. how do I know where, how many, and how large to place? I
have 32x10 in the hallway and one on the wall of our only vaulted
ceiling room. (ranch home w/ full bsmt) But w/o returns in the 3
BRs, they don't regulate well.