The HEPA filter used is for Roidmi F8Pro, search in your local online store. I don't include links since they may expire.
Price for the filter varies in different locations, the ones I got is roughly $1 USD/piece.
Actually there's still tiny air flow when the fan is at above 30%
Barely enough for keeping the print chamber in negative pressure, but it actually does work when printing ABS, making the smell much less noticeable.
The author remixed this model.
Just stick the original design with the official filter case model, making the adapter screw-less.
Snap the adapter just like the official filter box included in the K1C box and that's all!
Please hit like if it's helpful to you!