Many thanks to Wolfman Zen for this. This is the easiest and cheapest DIY filter I've seen, ideal for noobs like me. If this is already on gc I haven't found it. https://www.gardenscure.com/420/security/117667-best-diy-ez-walmart-carbon-filter.html
There's lots of similar things you can use. I have seen several different types/sizes etc but you need to check out a few shops. You can also be creative putting the carbon inside various plastic bottles etc....
You could just attach one of these strips infront of the exhaust fan, using a stapler. They're often found on litter boxes for cats, so just have a look inside your local pet store.
What are those stips called? Do they mask the smell of a flowering plant? And will it strain the fan? Thanks
They're just strips of activated carbon that you can find on quality litter boxes for cats. They're used to neutralize the smell of cat shit, and seing as they contain activated carbon, they should work just as well as the this homemade model. They come in different sizes etc. depending on the litter box. You should be able to find them in petshops, like I said earlier.