How to Make a Thermos

Cut off the bottom of your 2-liter bottle. Don't throw anything away, as you'll be reattaching it later.

Put the smaller water bottle (there should be about an inch of clearance between your water bottle and the walls of the 2-liter bottle) into the 2-liter bottle and cut a hole to accommodate the water bottle top. Because the top spouts on 2-liter bottles are small, you need to cut out the standard opening, ensuring that the new opening is the right size to snugly fit your interior water bottle.

Remove the water bottle from the inside of the 2-liter and wrap it tightly in aluminum foil with the shiny side facing inward. Tape the aluminum foil on. This will ensure that the heat radiating from the liquid inside will be reflected back in, rather than escaping.

Put the water bottle back into the 2-liter bottle and begin surrounding it with the expanding insulation epoxy. Coat the space between the water bottle and 2-liter bottle evenly and slowly, allowing the epoxy time to expand and get air into the mixture.

Squeeze some expanding insulating foam into the bottom of the 2-liter bottle. The foam will expand quickly, so you don't need much.

Once the epoxy has expanded to fill both halves of the 2-liter bottle, reattach the bottom. Use tape or glue to seal the 2-liter shut. You should now have a water bottle sealed fully inside a 2-liter bottle, with only the drinking rim of the water bottle visible above the top of the 2-liter bottle.