Hackers have been modifying toaster ovens to act as reflow ovens for years.

Nate Seidle himself from Sparkfun describes one example at: https://www.sparkfun.com/tutorials/60 . Sparkfun also developed a (now discontinued) controller PCB designed to make it simpler to modify a toaster oven. Link at https://www.sparkfun.com/products/retired/81

A more recent, but also discontinued oven controller can be found at https://github.com/botletics/Reflowduino

There was a PID controller made by "Rocket Scream" which is also discontinued. Hackaday user [Scott] has a nice writeup of his reflow oven using the Rocket Scream PID board at https://hackaday.io/project/18802-home-brew-smt-reflow-oven

If you prefer videos, here's one recommended by Hackaday in 2019: https://hackaday.com/2019/03/10/diy-reflow-oven-is-heavily-documented/

A web search will bring up many, many other examples. What I COULDN'T find though was an up-to-date PCB, that is in-stock, that I can use to control a generic oven. So I'll design my own, using a microcontroller compatible with Arduino. I will  be standing on the shoulders of giants with this project, which will hopefully speed the development and give me a better final product than if I were to design and build from scratch. A great starting point is the "Reflow Oven Controller" by 0xPIT on Github: https://github.com/0xPIT/reflowOvenController . That screen with the temperature graph looks great, and 0xPIT has also kindly included some very useful design notes on how to get the most out of solid-state relays, and some other circuit design notes. I can definitely work with this!

A couple of changes stand out at first glance:

• I'm not a fan of the Arduino Pro Micro (I prefer a discrete USB to serial bridge instead of the integrated solution on the Pro Micro) so I will use an Arduino Nano instead
• The Eagle PCB software was OK when it was published by CADSoft, but since the takeover by Autodesk, hobbyist users have really suffered. Autodesk are very hostile towards open-source and have a habit of locking your designs in the cloud. I'm not going to be a part of this and will instead port the parts of 0xPIT's design that I want to use into Kicad...