Recently replaced a 40yo 120k BTU gas furnace w/Coleman TM9V100C20MP11 (Specs: 95% w/2 stage vari speed ECM 2000 CFM).
The prior blower (which had been upgraded) is not as powerful as this new ECM. (For the curious, the old GE furnace with an iron heater exchanger had no cracks)
FYI: A licensed HVAC friend did the installation.

I also want to install an Electronic Filter. (currently "rigged" with a temporary 1" filter) I keep returning to Honeywell but also saw compelling comments for the AprilAire 5000 & Broan HEPA. (open to suggestions)

My concern is CFM, static pressure, air flow. The dimensions of the furnace sidewall is: ~18" height x 28" deep (the receiving air chamber) which will not accommodate the "properly sized" 2000 CFM electronic Honeywell filter which suggests 20x25. (the 16x25 is rated @1400 CFM)
The current (and temporary) sheet metal return elbow is 22 x 13.

I could not find any off-the-shelf bases that had a similar footprint as this Coleman that would allow the furnace to be stacked above this "theoretical" base so a wider return (and filter) could mounted from beneath. (the footprint dimensions would accommodate a 20x25 filter).
FYI: This basement installation is stacked on 16" concrete blocks as this unit was much shorter than prior furnace.

Also since this furnace has already been installed, I am not really interested in a bottom return as it would require a ghastly patch on the side where the return elbow is currently attached.

I was wondering, has anyone ever seen or created a DIAGONALLY mounted air filter?
I was thinking, I could have a local HVAC sheetmetal fabricator build an elbow that allowed the 20x25x? return grille to be mounted diagonally.

I appreciate any comments or suggestions. Thx