This whistle is custom made in aerospace grade titanium to fit in the slot of ToolFob. It is extremely loud and has no moving parts. It can plug into the Hex Slot on the Toolfob for easy lip alignment. It weighs in at 1 GRAM! There is no smaller and louder whistle out there.
Here's how to use it: https://youtu.be/6vUFZ0tzm4M