I bought this one on amazon: All-Filters CP-6005 Cut to Fit Carbon Pad for Air Purifiers https://www.amazon.com/dp/B000U204W2/ref=cm_sw_r_cp_api_i_Tqz3EbREN00F5 It’s a pain to cut it perfectly, I also had to fold it because it’s thinner than the original, but I think it works the same.

HEPA filter

Sorry for implying that AirOasis purifiers have HEPA filters when all they have is AHPCO cells that have to be replaced and steel filters that only have to be rinsed off with water. I've never used HEPA filters because no one in my household is bothered by particles large enough to be captured by a HEPA filter.

off-gassing

I can no longer smell the VOCs that bother me so I'm not a good one to ask about off-gassing. All I can tell you is: my VOC symptoms were gone within a few hours of installing the purifiers, but I don't know if that's because it took that long to purify the whole room or because the purifier finished off-gassing or because there's no HEPA filter in it.

charcoal bags

Increase the surface area of the carbon/charcoal that comes into contact with the air as much as possible if you want to reduce VOCs. Putting carbon/charcoal in bags reduces the surface area too much. You can put activated charcoal powder or *crushed charcoal briquettes (starter-fluid-free) in a bowl. Put the bowl inside a bowl and don't fill it to the top so you won't have so much mess if the bowl gets knocked over. You can also put a piece of carbon filter over the air inlet or outlet of a purifier or fan.