Snoopy
I have made small changes to the configuration. You mentioned that the cartridge was looked as if on the outside and it was. I was told that the filter itself is not directional and flow through it is controlled by valves in the respirator. But just in case I decided to mount it inside so the flow of “bad” air is through the filter is if it were being worn. These cartridges are available at Home Depot or Lowe’s.
Pic 1 is the inside and the glopped on silicone seals around the cartridge. Going for seal here not looks.
Pic 2 is a few screws hold it in place.