How do you make a thermos flask at home?

Materials

• Two plastic bottles: one larger, one smaller. (I used a 2 liter bottle and a 34-oz bottle).
• A sheet of aluminum foil about 24 inches long.
• Expanding insulation foam, such as the kind used to seal cracks around Windows and doors. (Warning: this is messy stuff.)
• (You might be able to substitute finely-chopped styrofoam packing peanunts; I haven't tried.)

Tools used:

• Razor saw, or a similar fine-toothed saw.
• X-acto knife or other sharp knife.
• Scissors.
• Plenty of rags and newspapers to contain the mess.
• Gloves, goggles, and other protective equipment