Sorry about double posting Mike's video, I didn't realize it was already linked to.
A kettle made for 120V (ac or DC, does not matter) is going to see 1/10th of the voltage at 12V, which will be 1/144th of the power.
Over time I've come to dislike the gold colored aluminimum resistors such as in Mikes YT vid. These are very heat sensitive, break easily, and are more expensive than ceramic resistors.
Ceramic resistors are pretty cheap, and you can easily mount 2 of them to a chunk of aluminimum. Just tap a single hole in the Aluminimum between them and use a big washer with a screw. More neatly also more effective would be to use a short piece of Aluminimum U-profile with a hole in the middle. The flanges of the U-profile provide extra cooling.
5W ceramic resitors cost about 15ct from Ali / Ebay / China, and I would not be surprised if these can safely & reliably handle a 10 fold overload, if bolted to a chunk of aluminimum.
If you want to go fancy, bolt them to a piece of square aluminimum tubing, and pump water through the tube.
These ceramic resistors can get bloody hot without failing.
A very simple & effective overload cutout would be to solder the wires close to the resistors, and ad a spring. If the thing gets so hot that the solder melts, the spring will pull the wire away and break contact.
Edit:
I just had a peek at a datasheet for a 5W Ohmite resistor, Series TWM/TWW:
https://octopart.com/tww5jr39e-ohmite-1147827http://datasheet.octopart.com/TWW5JR39E-Ohmite-datasheet-8450738.pdfThese resistors are rated for a temperature of 275 Celcius, and have to be derated lineairly from the ambient temperature. So they can be safely used to temperatures above the melting point of solder, and they have 2 nice flat surfaces to bolt some aluminimum on.