I have considered a little 12v kettle. This one is made for the car, plugs into the cigarette lighter, I think it heats to boiling in 15min. It seems safer to me than an immersion heater, but I have not used any of these items personally.

https://www.amazon.com/dp/B07FD5RMYN/ref=cm_sw_em_r_mt_dp_Rpr0FbTZN88AB?_encoding=UTF8&psc=1