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:https://www.instructables.com/id/Loud-Easy-Simple-Whistle-Bic-Pen/https://www.instructables.com/id/Make-your-own-whistlehttp://www.ggwhistles.com/howto/http://www.jonsbushcraft.com/elderwhistle.htmhttp://www.e-scoutcraft.com/wood_carving/whistle.html