A 2L pet bottle will become a fine hot water bottle.
Pour hot water of about 60~80 degree celsius to a plastic bottle.
If you can boil water with gas, the poured water should be about 60 degree celcius.
Find a heat-resistant plastic bottle.
Be careful not to burn yourself.
The bottle may distort when pouring the water.
The following is quoted from the link.
Mineral water bottles (especially foreign brands)
Bottles used for carbonated beverages (e.g. cokes) . They seem to be highly resistant to pressure, but not tolerant to high temperature.
Bottles used for liquid seasoning
●Check if there are any scratch and misshapen mouth.
●Before trying, keep the following in mind; this purpose is not supported by the manufacturers; all responsibility on trying this is on you.
2 liter pet bottle
Pot to boil water
Towels and gloves (cotton or rubber)