Air vent grille, gravity flaps, for 100mm duct (4 inch)

I made this cover for the vent in my bedroom. I noticed that my vent is not closed on the other side (the outside of the building,) which means that when the fan is off and the AC is on, precious cold air leaks out of the vent hole.

This print is similar to things like these that you can buy on Amazon: https://www.amazon.co.uk/Gravity-200x200mm-Ducting-External-Ventilation/dp/B00QOEYTBW

Basically you put it on your vent hole from the outside, and the flaps automatically open when the fan i