The stock filter isn't great and I doubt it is actually terribly effective in removing VOCs due to the very shallow bed depth of carbon, however if you wanted to get a replacement filter that is not external you would be out of luck as elegoo doesn't sell them. This printable version fits in the same place as the stock filter and allows you to use any granular or pelletized filter media. I would suggest overextruding the walls somewhat for strength. The top slides on but M3 countersunk screws can be added for increased security, or you could just use tape.