How to get better filtration in this system – GreenBuildingAdvisor

Seeking suggestions on how to upgrade my furnace/AC filtration to, say, MERV 13.

I assume a 1″ filter cannot get me there without excessive pressure drop.  Airflow is current utterly unknown, but I have a feeling it is on the higher side.

The current state of things is this:

 – 30+ year old furnace and AC unit

 – 1″ filter inserts right against furnace wall, held in place by tabs on brackets installed at top and bottom (see photo).

 – The opening in the furnace wall is shorter than the filter slot such that only about 70% of the area of current filters even do any filtering.  

 

Would there be any sense on attempting a MERV 13 filter without a dedicated cabinet?  My thought is much of its benefit would be lost to bypass if the “seal” is accomplished by a slight compression of the cardboard frame against the furnace wall.

If a cabinet is necessary, would be good enough to put it in the short horizontal section of duct?  It is a skosh less than 5″.  Or would it really only make sense to put it in the vertical duct with some transition before and after?  (see drawing in one of the pictures).  How hard would that be to DIY?

I am open to purchasing a manometer to ensure I do not introduce too much pressure drop.