Many years ago I had an activated carbon air filter system in the house, the replacement filter was a giant flat tea-bag maybe 50cm X 50cm (which would make a big pot of tea) full of activated carbon, and set in a flat, plastic cross hatch frame, I think they are probably still standard filter products, so just the teabag big enough and thick enough to wrap around the inner pipe and fill the space would be fine.
something similar to the pic,
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