I mentioned in the review of the Les Stroud MT that the whistle wasn't very loud, and said a pen cap was louder
I assume everyone knows this trick, but just in case you don't...
Any solid (no holes in it) pen cap can be used as a whistle. This also includes things like gun shell casings, although, the smaller, the better. .22lr casings are best.
I also came across this video of using a sharpie cap, so maybe even larger caps and casings will work if you partially cover the opening. Oh... and if you have no cap, but you have a pen... cut the bottom inch or so off of it, and it'll work as a whistle. And, in case you can't find a pen or shell casing, I'll post a link below the video on how to do it with an acorn cap.
So, there's another little trick to stick in your survival skill kit.
http://novelty-toys.wonderhowto.com/how-to/make-super-loud-whistle-with-sharpie-marker-266667/http://www.sciencetoymaker.org/acorn/assembl.html