What about putting a 12v heat pad in a cooler?

Maybe not this exact one. A little smaller would probably work better.

https://www.amazon.com/Cushion-Cushions-Universal-Supplies-Heating/dp/B07L1W5H7S/ref=sr_1_1?keywords=Heated+Seat+Cushion+for+Car-+Front+Seats+Auto+Seat+Cushions+Pad%2C+12V+Non+Slip+Car+Heat+Seat+Cushions+Cover+Pad+Winter+Warmer%2C+Universal+Fit+for+Car+Trucks+Auto+Supplies+Home+Office+Chair+Pet+Heating&qid=1570584019&sr=8-1

or

https://www.amazon.com/152X304mm-Silicone-Electric-Thermostat-Delivery/dp/B07QD4GSMY/ref=sr_1_1?keywords=152X304mm+100W+12V+Silicone+Electric+Heating+Pads+with+65+Celsius+Thermostat+Heater+for+Pizza+Hot+Food+Delivery+Bag+Mat+Heating&qid=1570584838&sr=8-1

Edit: Some issues with this idea are the fact that most of these heat pads have 20 minute timers in them so you might need to keep resetting it (this might not be an issue with the pizza heater). Also the whole thermostat thing might be a little harder to implement. But with some knowledge of electronics they could be worked around.