I like messing around with gear that is very compact yet still functional.

One of the smallest functional whistles I've seen is the REI small keychain whistle. It gets so-so reviews since sometimes a batch of them is not made right. I also thought it better for a whistle to be plastic instead of metal, in case you use it in winter.

I saw one of those backpack sternum strap buckles with an integrated whistle and figured I could miniaturize it for a super compact kit.





I used a Dremel's cutting wheel and grinding wheel to remove the nonessential parts, leaving a tab for a 3/32" lanyard hole. The plastic is not brittle and it's easy to cut on. It worked out pretty well, and the whistle is darn loud. Not bad for about $2.