This Instructable shows you how to transform a common ballpoint pen into a survival whistle which will fit inside an Altoids tin and still work as a pen. This multi-functionality helps to save space in the tight confines of the tin.
DISCLAIMER: Though this whistle is ear-splittingly loud when configured properly, I have not tested it in a survival situation. Please use your own judgment on whether your end product is good enough for a real survival situation, if that is its intended use. If it isn't good enough for that, please consider shelling out the money for a factory-made whistle.
Here are some pages I viewed which were very helpful in learning about the construction of whistles: