This flap is used to dampen the noise and air coming through the poop hole a bit.
If you are using a HEPA filter for your exhaust fan, still a lot of air is pushed out the poop hole, so that can reduce that.
Also, the noise difference was slightly hearable, so if you are currently trying to reduce the noise of your printer, that could be an extra accessory.
Downside:
The first poop can't pass it, so there is still a bit WIP to make it possible. Maybe try to reduce the infill to 5%.
It could also be thinner in general, I primarily made it thicker to avoid needing supports.

Bills of Material:

• 3x 6mmx2mm magnets
• a piece of filament.

Assembly:
For the hinge you can use a piece of filament, the holes are a bit bigger, so the flap can swing freely, and it's easier to stick filament in.
The magnets should stick without glue, just push them in with a bit of force.