I wouldn't use a travel kettle heater. They're designed to...
1) heat water up to 100C.
Oxalic Acid needs a temperature of at least 157C to sublimate, any cooler it just won't work, any hotter than 189C and you run the risk of decomposing the OA into other products. Do you really want to risk that?
It might not even reach 157C, or do so intermittently.
2) be immersed in water in use.
Using them dry may produce hot spots across the element leading to its failure.
---
Glow plugs have the same problem in that you can't be sure of the operating temperature. Not a big problem but they do draw a lot of power, so if you need to do multiple hives, then it makes sense to use a device that will heat as much as you need to make it effective, but not too much more, so that your battery will last for the maximum number of treatments.
Much better is to use a 12V PTC heater which is temperature regulated to stay at exactly 170C or 180C.
These will provide the optimum operating temperature while being the most frugal with battery power. They can be quite cheaply obtained from the usual auction sites.