The constant in the equation is the power board. It's the one piece of the project where all the hazardous parts are concentrated, and I don't see any particular need to fiddle around with its design.
The controller is another story. Some folks might want to try to build the controller into the oven, some might like the "backpack" controller concept, while some might want to use their own i2c display backpack, which means a separate controller. Others might want to breadboard it (like I did with the original prototype), and still others might want to build theirs with an Arduino Uno so they can re-use the Uno for other things.
Once you have a converted toaster oven with the power board installed, there are only three building blocks remaining of any consequence: the thermocouple interface, the microcontroller itself, and the user interface.
For the thermocouple, you need a thermocouple amplifier and a K type thermocouple. My original prototype used an AD595, but that chip is quite expensive. Analog Devices makes a newer part that does the same job at about a third the price - the AD8495. The only problem is that that part is an MSOP, which when you're building a reflow oven represents a chicken-egg problem. I've designed and now offer for sale a solution in the form of a break-out board. It has the AD8495's reference design constructed with SMD parts with a 3 pin SIP header for convenient mounting on a breadboard. Simply supply it with power and it will output a 5 mV per ºC voltage - perfect for feeding into an Arduino analog pin.
As for the oven, you can simply connect a 220 ohm resistor to two digital output pins and then to the power board. Setting the pins high will turn on the respective heating element.
The only part left is the UI. The Toast-R-Reflow firmware is designed to talk to the AdaFruit RGB shield (or OpenEVSE RGB LCD backpack), but you could instead use serial I/O and the Arduino serial monitor to do the same job.