This is a fantastic idea! I really dont have the $80-100 to spend on a premade can filter and it would be a downright pain in the ass to hang up in a grow thats already running.
Im headed to the store tomorrow get the materials needed and tape a filter down to both sides of my exhaust fan to make sure it gets scrubbed twice. Ill double check all of my seals for the venting flaps to make sure no smell can get out.
Excellent guide! +Rep to OP