I always wanted some kind of "collar" that I could put around a burner to try to direct that excess heat back into the pan/pot.

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.

minor side effect

Death only increases the amount of carbon emissions this method saves.

Product idea: a broken CO detector that saves up to 100% of person’s future emissions. Saves money in the future too.

Lower carbon emissions with this one weird trick!

But your decomposing family will release CO2!

It's offset against the future CO2 emissions of the family and their descendants.

(Gee, this thread got dark. Like carbon-black dark)

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.

Typically, those collars are used as a wind screen.

https://www.rei.com/product/139472/toaks-titanium-windscreen

No - found a reference:

https://www.msrgear.com/stoves/stove-accessories/heat-exchan...

This is maybe useful for boiling water but not so great for cooking. You want your pot to have a relatively uniform temperature, and gas burns very hot. This means that, especially if the walls of the pan are thin, any part of the wall that has good thermal contact with combustion products and poor contact with food will get extremely hot, with various unfortunate consequences. Also the handles will get hot.

Well it takes absolutely forever to boil water on a camp stove, so I"ll take it.

Look into jetboil campstove/pot. It's got a heat exchanger fin stack on the bottom of the pot to improve boil time.

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