npm install worked for me as well, somehow npm rebuild didnt do this job.

Drill holes in the top and in the bottom for a chimney effect. Heat goes always upwards.
You can check your temp easy with MMM-stats, it tells you the temp of your cpu and uptime and so on

An important step is to choose a good mirror. Thats step one. Knowing its transmission is important to the lightlevel of the place where the mirror will be located. This means how much light comes from the monitor through the mirror and is visible for example in you bathroom. A lot of german users recommend this product:

https://www.glas-star.de/spionsspiegelnachmass/chrome-spy-spiegel/chrome-spy-6mm-nach-maß/

You can set your own size.

The backside of the mirror can be mdf, but it should be painted black. This raises the mirror effect and no light shines through. You can also build a small frame and spray the uncovered parts black.
For glueing you can use mirror glueing tape or a hot glue gun. This results are most stable. Glass weights a lot.

Be aware of the right ventilation for your pi. Otherwise its getting too hot, even if you use a fan.
Hope I could help a bit.

Depending on what kind of mirror you are using, you can choose a lot of solutions from this forum:

https://forum.magicmirror.builders/category/12/show-your-mirror

A lot of articles tell you exactly what kind of glue or mechanical solution they choosed

Please raise your question in english so most of us could understand it