Electric haybox

I'm fully aware that's a weird topic title, but still exactly what my first post is about.

Before putting food in the haybox it has to be heated for a short while.
Would it be possible to do the heating inside the haybox.
I've been thinking about a heating pad/wire that runs on electricity.
Preferably solar powered or from a battery.

It does work if the power is high enough, but my aim is low power.
So... my question is if someone has experimented with this type of setup?
When cooking on electric the power usage is very, but that for a large part because the heat escapes all the time.
So let's say if a haybox is heated with 25 watt, would the trapped heat eventually rise enough to for example boil rice?
I would say yes, if the box is very good at retaining the heat. But such a box may be not practical to build.
Things like steam inside haybox likely aren't a positive either.

Hints, tips or suggestions?

So you want a super insulated, low power, hyper efficient, crock pot.  Sure it can be done, I would not use actual hay anywhere near the heat source.

If you are only doing thin liquid broth or soup, a simple immersion heater would work.  But it will burn out if you use it with stews or oatmeal or beans.

I have a heated mattress for my dog that is only 7 watts, it would keep food warm a lot longer but wouldn't warm up something from cold.  You would need a lot more power initially to do that, but it could maintain temp on almost no wattage.

McMaster has silicone heat strips for dc and ac low wattage applications.  I use a 25w @ 12v to keep a chicken waterer from freezing.  I have found these type of designs need a supper amount of insulation.  

@ R. Scott
No hay in my haybox for sure. I have several ideas. A small freezer box. Or a wooden box lined on the inside with quality insulation. Then line that again with an inner box.

I like your crockpot idea!
Using a 12v crockpot (do they exist?) is a very good starting point.
It's solid, it works, hygienic, and safer than messing with heating pads etc I guess.
Then the last step is trap the heat that usually goes into the kitchen in the haybox.

I found a 12v crockpot that uses 78watt. I'm new on this forum so I don't know I'm allowed to post links to external pages (spam)
78watts is still a bit high, I wonder if there are lower power offers.
I'm aware lower power heats up slower, but it's easier to find matching batteries/solar power for it.

Christopher Shepherd wrote:McMaster has silicone heat strips for dc and ac low wattage applications.  I use a 25w @ 12v to keep a chicken waterer from freezing.  I have found these type of designs need a supper amount of insulation.  

Are they heat resistant?
That  may sound like an odd question to ask about an heating element...
I once had battery powered insoles to keep my feet warm. They worked.
Then, for  a reason I forgot, wrapped them in a few heavy towels.
A while later the soles shrunk to half size and where all curled up.
My guess they are designed for body temp plus a few degrees extra. But wrapping them in towels overheated them. Those towels were acting as a simple haybox.

Yes they are resistant heat.  They can heat up to 490F and can work in an environment at 300F.