A week ago I ask here for help, because my Chromecast ultra became too hot and runs unstable.

Now I want to share my solution. It works fine for me and I hope it helps other people, too.

I bought a 40x40mm heatsink. This one: https://www.amazon.de/dp/B087333C1H/ref=cm_sw_r_apan_glt_i_DYAH2NQ8HMER03GN6751?_encoding=UTF8&psc=1

But it only came with duct tape, not thermal pad / tape, which is better in my opinion.

So I removed the duct tape and the glue with the help of label cleaner / brake cleaner and replaced it with a thermal pad. This one:

https://www.amazon.de/dp/B00UYTTDO6/ref=cm_sw_r_apan_glt_i_53HMC2AKRE5R32TE2TTS?_encoding=UTF8&psc=1

It's 50mm, but you can easily cut it to 40mm with a scissor.

Important: The heatsink have to be fixed to the glossy side.

I've done a picture:

https://ibb.co/Ld30QnH

Optionally you can additional buy a usb-radiator.

Remember if you have stutter problems, it also can be a problem with your network hardware. Restart the access point / router often can help.