Once I had apart, I took a look at the insides. Overall, the design is very simple. These new models have moved on from incandescent bulbs and now use a nichrome wire as the heating element. The nichrome wire is connected through a switch and across mains. Part of the nichrome wire is used as a resistor divider to power the light. The light assembly has an LED, some current limiting resistors, and a Schottky diode anti-parallel to the LED. The diode/LED pair receives a stepped-down AC voltage when the heating element is on.

The major modification I had to make was to the heating element. USB-C spec allows a maximum voltage of 20V, whereas mains is closer to 120V. Rather than step up the USB-C voltage, I instead added some wires to the heating element to divide it into six equal segments, then wired all six segments in parallel. This lowers the resistance by a factor of 36, so at one-sixth the voltage, the current increases by a factor six. Thus, the power is about the same. It's hard to solder to nichrome wire, so I just wrapped the copper around the nichrome at least once at each junction. After my modifications, the cold resistance dropped from 190 Ohms to 6 Ohms, which means I wasn't perfect in dividing up the nichrome wire, but it should be close enough. I tested the newly modified element on its own with a bench power supply. At 20V constant voltage, it was drawing about 3.75A, or about 75W total. This was a bit lower than the rating printed on the case of the oven, but it's good enough for this demonstration.

I made a few other minor modifications. I changed the LED assembly to run off 5VDC by replacing the resistors with a lower value. I also epoxied my USB-C breakout board onto the metal panel that previously held the strain relief for the mains power cable coming into the unit.