Help me build a whole house carbon filter inside HVAC stack

[Android]

Because I exhaust to HVAC return stack already and converting one more room to grow room, I need to install the following:

There are standard HVAC filter housings available for sale.
Usually they get installed vertically right before the furnace, but I want to install one in the return stack horizontally.
After that, I would need to fabricate a simple tray with fine metal mesh on the bottom that would be filter-sized, would slide inside the housing, and would hold pellets of activated carbon (~5#)

Has anyone tried/done anything similar?

I will draw pictures and put them up in a bit

 

[dadums]

You can buy furnace filters that have carbon imbedded.

 

[Android]

I was only able to find bulk pre-filter media (black porous material), but no actual furnace filters.
Do you happen to know the brand name?

Sidenote:
I am also thinking of ways how to utilize the existing filter housing - that would have been much easier task to accomplish.

 

[dadums]

I don't know the brand, but home depot or any big box hardware store would have them

 

[Android]

I've just bought a case (dozen) of carbon furnace filters online for $115.
Does anyone here uses them?
How good of a job are they doing?