The safest thing to use is glass, like a glass jar. I save used glass jars/bottles to use for food/drink like hot tea. Do they make glass thermos'? If you find one let me know!

A metal thermos would be next best, like stainless steel.

Forget plastics for heating stuff in. PVC and methacrylates leach DEHA's and bisphenol-A xenoestrogens, especially when heated. Polyethylene is safer (supposedly). Some plastic containers have the recycle codes with a number and letters which tells what it is made of. Most Nalgene water bottles are polycarbonate - bad stuff. Plastics are even worse when you use detergents to clean them.