Inline backdraft protector

We had a draft coming in through the over-stove microwave vent.  This is my solution.   I thought I would post it in case anyone else found it useful.

This stops air flowing back into the house, and also doubles as extra insulation layer reducing the cold-air conduction.   Simply remove your outdoor cover, stick it in (make sure it's the right direction, and right way up), use some caulk to seal the edges, and put the cover back on.

DO NOT USE THIS IN A DRYER VENT as the lint and heat could lead to some safety concerns.

I printed in ASA, though I think PETG would be fine as well.

The flap is attached to the main housing via two finishing nails.  You could get away with a cut paperclip and some glue as well -- as long as it's low friction, and metal, it should work.

See pics for how things fit together.  If you squeeze the top and bottom of the housing, the hinges separate making it easier to insert the flap.

This is for 4" piping.  If you need another size ping me -- this is just a single parameter in my model, so I can adjust the size easily enough.