Still interested in January?
One of the best materials, and probably the most practical, would be to add an ice cube in the container together with the coke can.
Then the thermos bottle isn't absolutely necessary (though it would avoid the ice cube). 2cm walls of polystyrene foam would let the ice cube melt within one hour.
A thermos bottle can be produced for cheap, of injected plastic. I'd take a resilient plastic for safety (not glass), like polypropylene, which is also weldable. Inject an inner wall and an outer wall, metallize them, then weld them to one single bottle. Either weld within vacuum, or evacuate after welding, through a small tube of clean not-outgassing material closed by melting.
In both cases, you need a very tight cap, which is the difficult part of the design.
I saw this question on many forums... Probably the result of a market analysis.