Lol pay $300 for something that cost you maybe $50. I use a 1 inch think layer in mine and it has a massive surface to make sure the draw is slow to fillter out near any smell.
I used 3 laundry bucket for mine. 2 buckets have the arse's cut out those ends are the base for the filter. It has a 1 ft x 8 inch dai outtacage, 1 ft x 6 inch dai inner cage. The inner cage has a outta fly screen metal cage and the outta 8 inch dia a inner cage of metal fly mesh. 1" from the top is filled with hot glue to componsate for the activate carbon settling and sinking over time so no air can pass through with out being passed through the carbon. 3 thread 1/4" rods pass through the centre to hold it togher. These rods also go up into the top bucket where a 25cm dai desk fan is mounted onto them which sits in a bucket. This acts like a inline fan. My srubber rocks is all i can say and i can replace the actiavte carbon as soon as i feel its not doing its job for a cost of $15.
The key is to have a large surface area with a good thinkness of filter carbon, this makes sure the carbon can remove the smell.
The next one i make will be a cube setup up like a evopartive air connditioner using an inverted ceiling fan XD