The nevermore flex can be found here:
https://github.com/nevermore3d/Nevermore_FLEX
I like it more than an integrated filter, because it has much more space for activated carbon.
Also it can be used to exhaust the air outside, if wanted.
For the k1 exhaust I used hot glue.
I used 42mm inner diameter aluminium flextube.
The author remixed this model.
Adapter added to create a closed loop filtration system with the nevermore flex.