There is a Zojirushi mug (you can get from amazon.ca for around $25), keeps warm for over 8-10hrs. People use that to make congees too. You throw in some rice, fill with boiling hot water, wait 3-4 hours, a congee is done. No electric outlet is required.
Other alternative is,
1. Learn to make congee with microwave (bring some frozen cooked rice)