Hey arbesfeld, I really like that project, and the nice writeup really presents it well. But especially as people might use it for inspiration for their own temperature-controlled cooking projects, I think the lack of a proper isolation of the mains-powered parts is a little bit worrying.

For the next project, or maybe as advice to the random person looking at the HN comments, here's what I normally do:

Try to minimize the area of unisolated conductors in devices on which you work. If I solder cables to e.g. a mains inlet, I'd put heat-shrink-tubing over it, so that it's not touchable anymore. Or use the proper crimped terminations, which also are available in isolated versions.

https://www.google.de/search?q=flachstecker&tbm=isch

To protect braided wire, and make the connections more reliable over time, I like to use "wire termination" (I don't know the correct english term, in Germany we call them Aderendhülse (literally: conductor end sheath).

https://www.google.de/search?q=aderendhülse&tbm=isch

Instead of these "screw on caps" for cables, try to find suitable terminal blocks.

If you have the possibility to access a 3D printer, make a small clip that goes over the screw-terminals of the solid-state relay.

If possible, try to have a "high-voltage" and a "low voltage" side in your enclosure, that way, you are even less likely to come in contact with mains voltage while working e.g. on the MCU part (connecting/disconnecting the programming cable).

I'd like to stress again: I really like your project as presented, I only want to show some ways in which the project could serve maybe as a inspiration also to safe mains-powered electronic design.