Have you heard of hay box cookers? Very useful for rice and pastas especially but almost anything benefits.
To do it easily, boil some rice in water and pour it into a thermos flask. When you want to eat later in the day the rice is cooked and hot. The flask keeps it all hot enough to cook the rice.
You can even just put the dry rice into the flask and pour boiling water in. Give it a stir, put the cap on and thirty or forty minutes later you've got cooked rice.
It takes longer off the boil but it still cooks through very well.
White rice cooks in 12 minutes slowly simmering. Brown rice takes longer.
You can cook veg in a vacuum flask as well, but meats need cooking a bit before going into the flask. Wide-mouthed flasks for food are easily available.
I have an ex-Army one that holds 2 gallons and cooks a meal for 6 people easily.
A 2-litre one goes in my pack on some trips. Usually the little Dragonfly stove from MSR does for my cooking but quite often on long cycling or walking trips my lunch is cooking on the way. Nice.
Make breakfast and then get lunch cooking.
Into the flask, into the pannier or rucksack and the portable flask cooker is working all morning.
You can make boxes with polystyrene linings to do similar.
Originally hay was used, hence the name...hay boxes.
With a large volume of boiling water to start with it stays hot enough and meat and veg can cook very well through a morning or afternoon.
Vacuum flasks make it possible to do with a smaller amount of food that would cool down too quickly in hay boxes.
The slow cooker idea is similar.... Low heat, long time. Good for cheaper cuts of meat or for cooking a whole chicken or a stew or casserole while you're at work or college all day.
Now, having got that done, if you can come up with an improvement or some feature that makes your product desirable you're on a winner.
Remember the saying attributed to Ralph Waldo Emerson, a famous essayist and philosopher from the USA?
"If a man builds a better mousetrap the world will beat a path to his door"
Well it wasn't quite true for what he really said, but the idea is right.
. . . .
This one is fun and a good laugh about inventing mousetraps, or anything else actually. Just reading the headlines is enough for a laugh.
. . . .
Of course if the food is already cooked and you just want to keep it hot, well that's been done too.
You get hostess trolleys that work on electric or with meths or gel burners in, and small ones for restaurant tables where there is a central dish to keep warm, with gel burners or tea lights (small stumpy candles in pots) under them.
For outdoors any insulated box with a burner in it will do the job.
A catalytic burner may be an idea for you to use. It doesn't have flames so it's safer.
It converts propane or butane to CO2 and water as with normal burning, but using Palladium or similar as a catalyst for the reaction and the top plate gets red hot like a normal burner when the gas is turned high, and you don't even need matches or a lighter.Turning the gas down reduces the heat and you can run them low enough to be just warm.
Catalytic burners are used for some hand warmers and very good they are too. Keep it safely in your pocket with no flames and wrap your hands round it to warm them up. Some hand warmers use a slow-burning stick of fuel and also work very well.
Whatever you design, it would need some different feature than is already available, or an improvement on it.
What about a collapsable one? Some already are but perhaps an elegent design or new folding method could be thought of. Modern insulators like aerogel are extremely light, so a folding aerogel catalytic heating oven (FACHO) might be OK.
Not a good name for slimmers though.