I'll leave your question to be answered by bamacool.
just wanted to tell you that media filters can be used in the
return air filter grill you have. without having to change the
r/a filter grill to accomidate media filter.
check with hvac supply house, or with your hvac company
for filters like the one in the pic attached.
if you to upsize the return duct & r/a filter grill..
use the filter grill that is made for media filters.
I use 20x25x4 for my 2 ton system.
best of luck
The cure of the part should not be attempted without the cure of the whole. ~Plato