I would use the 20x24 Return Air Vent..

I would want as larger a filter as you can get; never use a 1" thick pleated filter; too much pressure drop.

It would be best to have two (2) Return Air filter racks in the conditioned areas of your home.

You need 325-sq.ins., of filter area per Ton of cooling; 1138-sq.ins.,I'd use two 26x26 filter rack areas in the conditioned areas of your home.

That will keep the air velocity though a new filter under 300-fpm; that is imporgtant.

After further research and finding the data sheet for my unit, I found that my unit requires 720 Square Inches of filter to operate efficiently.

I have figured out a way to give it the filter it needs according to specs.

Thanks for your input~~~~Ron