Stainless Steel
like many existing flasks stainless steel was the was the obvious material requiring sufficent strength to support it internal space for a vaccum to contain the heat.
Wooden Shell
To give the flask a softer more natural feel the wooden outer proves a layer that can mark and give the flask a character and become something to cherish.
Silicone Liner
With Silicones ability to repel water and heat resistance it would be extra protection for the wood and emphasise design.
Silicone Strap/ handle
needing to secure up the side of the the flask without a separate clasp and still be able to curve back on its self to make the handle, Silicone was the perfect material.