The shape helps to keep the stopper in place, even if the window is moving due to wind. It can be used symmetrically.

I printed it in Armadillo TPU (https://ninjatek.com/shop/armadillo), a very hard TPU, but I think any material can be used.

To prevent damages on your window, while trying to close it with the stopper inside (this will happen sometimes at the beginning 😉), I recommend to print it in any kind of TPU.

It is 4cm wide at the thinnest point


UPDATE:
I made the outer wall a little bit thicker, so that when printed in TPU, it won't squeeze together but rather slide a little bit better up and down (hopefully)