Hi Katie,
I use the small Sigg Box for an intermediate sized survival kit. The Sigg Box is a bit thin walled but does work at boiling water with the Esbit stove pefectly well without buckling despite Siggs warning that it should not be used in his way. I have recently spotted a cheaper hard anodised type copy, which looks a bit more durable and has a folding stainless steel handle similar to a British army mess tin, but has a waterproof sealed lid such as the Sigg.
