The problem is you need airflow or you generate carbon monoxide instead of carbon dioxide. So simple solutions can get more heat but also kill you.

There used to be (still is?) a sort of vertical chute/collar available to put around camp pots. Think mountaineering applications. The idea was that they would capture heat energy escaping off the bottom of the pot that would rise/disperse into the environment, and channel it close to the side of the pot where it would heat the vessel as it rose. Think of wrapping a section of corrugated cardboard around the outside wall of a pot. Now a birds-eye view down the cardboard should reveal the energy capturing channels that will allow the sides of your pot to heat the pot contents. Adjust your material and tune the sizing and you’ve got a camping gadget.

Old cast iron skillets had a collar under them to better fit on the pot belly stovetop.