Don't you have any clues?

Use a heater and a controller.

Doesn't sound like rocket science.

I have a suggestion: You will need a heat source.

What will you be baking at 80° C? Is this a drying oven?

yes, it is a curing oven, and for the heat source I am interested in electric instead of gas burners, because of I could regulate the temperature with a PID controller, but I am not expert in where to put the heater and the heat distribution

Thats a large oven,

You know you could give more information, as to what your oven can be used for.

is it a batch oven?

I suggest Convection,

Decide on a Heating source, Gas, Electric, I suggest Gas. Insulation try 8# mineral wool, may be a good idea to clad it.

hows that for clues?

More clues.......

ok I read the rest, so its a curing oven. infrared, would be a start. What type of untiliies available?

Also be a good idea to pick up the latest addition of NFPA 86 Standard for Ovens and Furnances manual. I think that would be 2010.

The heat will transfer slowly through the steel of the container walls. However the container itself is a large thermal mass itself. Without insulation between container walls and your oven space this steel will require a lot of energy to raise the temperature by +50°C.

I strongly suggest that you consult or hire an electrician or electrical engineer for the control circuitry and/or heater to properly meet your local wiring code. You want an oven, not an out of control furnace.

FYI,

Just checked the temperature of my 20 foot pod, in the sun. Ambient temp is 98°F. Temp of the inside of the roof is 126°F.

Checked again. Ambient 101F/inside roof 145F

Boy, am I ever glad I don't live in a storage pod!

And it's not even noon yet!

I'm just grateful I have a parking spot in the shade.

You'd be amazed at the lengths people will go to to park in the shade here.

We issue dozens of warnings (and citations to repeat offenders) to people who try to park all day in the shade, in reserved or timed customer spaces.

As per illustration, a fan to circulate the air will be a good idea, and as already stated insulation.
Del

A cool breeze helps. This was today.

The pool was clean.

We need to know the specific heat of the material to be heated, its entering temperature, and how fast it must reach the desired temperature.Also significant is the ambient temperature outside the container.

hot water pipe around container with internal vent (you have to build working mass)

Here's a good starting point...

temperature control achieved by opening and closing the doors.

Have you considered safety. e.g. if anyone accidentally were to be shut inside an
obvious and easy way of opening the door from the inside could be a life saver. Also,

Hot spots? For such a large volume would it help to have a fan or blower circulate the
air (heat) more evenly? (else some could come out half, or over, baked?)

Depending on your location, and how competitive you need to be, I would definitely
compare the cost of oil / gas / electric supplies as there can be a huge variation
in running costs. (profitability) Why throw that away just for an easy install?

Have you fully considered the site? When up and running (getting warm) could it be
better sited elsewhere? Either to benefit from the warmth or, save other property
getting to hot? Just a few thoughts to help.

jt.

A man goes to a dog breeder to buy a guard dog only to be presented with a scruffy
terrier mongrel. "What use is that?" he asks.

"Ah but he's a trained killer, this one" promises the breeder. "Watch: guard dog? That chair!"
In a blur of little snappy teeth and yapping the chair is reduced to splinters.

"Amazing!" says the man "Can I have a go? Guard dog? That box!" In seconds the box is shredded to mere fluff.

Delighted the man buys the mutt and rushes home to show his lady wife. "Look at our new guard dog,"
he says. "He's a trained killer!"

"What that thing?" she scoffs at him "Guard dog my arse!"

(Because....People who smile live longer.)

What's the mass of that which you wish to cure? Depending on whether it's something thin and light or thick and heavy is the place to start.. if you have room to use an interior insulting wall and ceiling (as well as the entire exterior of course).. You will save energy. Fan/s a must... You can literally cook with lightning or found heater elements.. at that temp.. figure it out.. depending on your budget and usage rate? ... find balance.. ...or go out and have one built.. ;)