This is a Fully 3D Printed version of the popular “Eclipse” vent mod. Finding the correct vents for sale can be troublesome in some regions, and they are not exactly cheap. The flaps are strong enough and can stay shut by themselves even under vibrations. The linkages are strong enough, and don't experience lateral loads so they probably won't break under normal use. The vent body rotates inside the dash opening as well. My model has the added benefit of not requiring any tools to take the vents out for maintenance. They come out with a stronger pull.

Instructions:

Use PETG or ABS or ASA or stronger material with better heat resistance.

Print the flaps solid, but you can leave the body hollow with infill.

Hinge holes are modeled 1.9mm. I had to use a 1.75mm drill bit to clear them out. If you are confident with your machine's tolerances, you can try not doing that.

Use pieces of 1.75mm filaments as shafts for hinges. Insert them in place, leave 1-2mm poking out, and use a heated piece of metal to deform the head and fix the shaft in place.

Use brims for the vent body, since it has too small of a contact surface to stay stuck to the plate.