When you think of all the metal that is in contact with the whisky before it ends up in your bottle, I don't think you have to worry to much.
The stills is made out of copper. The tanks that the wash is in is made of stainless steel in many places. The tankers that transport the whisky from many distilleries is made of stainless steel (here we talk up to a week in a tanker). And there is also vatting tanks made of stainless steel.
So I think you can store whisky in a stainless flask for a week without worrying.
I use 6, 10, 15 and 20 cl glass flasks though. They're quite unbreakable. Made out of brown, thick glass. Medicin-type.