Made from a big water bottle wrapped in several layers of cardboard. The whole thing is covered with brown paper, which makes the surface harder and look nicer.

The lid is made from 4 layers of cardboard and trimmed to tightly fit the container. When used, it's covered in a plastic bag for waterproofing

Shaping cardboard to tight tolerances is pretty easy: when dry it is suprisingly stiff; you can cleanly saw it through with a serrated knife, even at tiny shallow angles. You can also add more layers of paper to make it thicker.

Is this to conserve electricity? I don't understand why it needs insulating, mine can handle gallons of water uninsulated.

Yes, it's to prevent heat loss and water loss. I just feel that an insulated container would lose a lot of heat over the hours of cooking.

What are you heating in this contraption?

Wouldn't the bottle basically contaminate your food when heated for hours? Would it be better to use something safer for the inner container?

Sous vide bags containing the food I want to cook

The inner container doesn't com into contact with the food. They are sealed in their own bags

I love that you made it out of cardboard. That's a reasonable material for the job, easy to work with and the whole thing has a great re-use/recycle vibe to it.

Ah, true, stupid me.

I have a sudden desire to turn my 21L steel stockpot into an extremely large insulated slow cooker.

Previously thought of using a straw box or similar where you heat it and then stick it in the box and seal it up and just leave it.

Now I just need to find enough people to eat up to 21L of stew or curry.

I did think of insulating my big pot too, but I don't use it very much. One time I made a big pot of spaghetti sauce for freezing and now I still haven't finished eating it.

Do you plan on running it outdoors in the winter or something?

When cooking for hours it makes big difference if you are using tens of watts of energy or hundreds.
And even in summer you would rather not want to heat your room together with the food.

Yeah there is that too, would need a lot of freezer tubs. I also don't have space for a microwave to reheat them. Some stuff could go in the air fryer, or a saucepan.

My partner really doesn't like eating stuff that has been frozen though. I think she gets it from her parents, they are really funny about it for some reason. They appear to think "best before" means "toxic after".

I would be pretty interested to know how much of a difference it ends up making after a long time in total energy usage. Could make a nice thick shell.

What was the process you used to shape the cardboard layers?

I cut them with a serrated knife or scissors, then glue the pieces together

I have some similar things with old coolers. I love doing chuck roast or pork shoulder for 48 hours. So it's nice to have something insulated with a lid.

Yeah, sometimes I just let it sit there to cook. The insulation would retain the temperature for a while