Wouldn’t a separate, super insulated container ultimately be more efficient and lighter? Just heat up the water near boiling and quickly pour into the super insulated container?
Awhile back, I was working on such a design that is very lightweight, using a combination of balsa wood and food grade silicone primarily for the container part, and then goose down + IR reflector (and EVA foam bottom) for the insulation.
The silicone coating would cover the inside of the balsa container and come up over the lip and down some. The rest of the balsa container would be reinforced with a combo of epoxy and thin EVA foam (so it could take rough handling. Then it would be wrapped in a fabric cozy made out of a combo of WP fabric (inside) and breathable fabric (outside) with goose down in same (and as mentioned an IR reflecting material as well).
I unfortunately had to put the project on hold, because I ran into some issues with cutting the balsa (stuff’s a pita), and need to invest in a good jig saw and special blades for same to complete the project, and money is almost always tight around here. Up until recently, time was tight too, as for the last 5 years or so until very recently, I’ve been working both full and part time jobs concurrently (and one of my semi recent full time jobs, sometimes required 60 hours).
But I’m down to two part time jobs and have more time and energy that I’ve had in awhile.
Anyways, one could easily beat the weight to insulation ratio of a typical SS thermos using such a combo of materials.
I’ve also though of using two epoxy/composite reinforced bamboo containers and pulling at least a partial vacuum in between them (and maybe also putting aeorgel in between them), but I don’t know if I trust even food grade epoxy long term with boiling water. You might be able to use silicone in the inside of the inner container, and just reinforce the rest with epoxy/composite.
Anyways, with a combination of composites and systems approach, you can definitely get some rather light weight, strong, durable, and very highly insulating containers for high efficiency cooking. My biggest hurdle in designing things, is almost always lack of extra spending money.