Whats the simplest heating surface for a DIY project
I am not familiar with that kind of devices but I don't think they are much different from e-cigarettes. Specifically:


They probably do not keep heat constant, but rather activate it when necessary, either manually or by air flow detection;
The temperature is sensed by measuring current and voltage and using thermal coefficient of the heating element material. That is the reason you have to configure it in the vaping mod when you change a coil;
The temperature control is usually quite primitive, no PID, just plain current limiting when coil reaches target temperature. Something like P-only control;
A single lithium battery with high current rating is typically used. This simplifies charging from USB port, but requires reliable low voltage and over current cut-offs;


So, using off shelf vaping mod as a base of your device could be a good starting point. You just have to figure how to replace vaping coil with suitable heating chamber. For example machine it from aluminum or stainless steel, wrap with something like polyimide film, then wrap coil around it and enclose everything into protective case.

Or you can find old or broken commercial IQOS and see how it is made. In any case some (probably a lot) of testing would be involved in project like this. The trick is to maximize area of contact with hot surface and minimize heat loss to the outside.

Finally, if you allow me going a little off-topic - whatever the delivery method is, the end result is not good for you.