I made a stashbox that will fit with this super loud whistle. It can not be opened while on a keychain, but the whistle is functional and VERY loud. Keep some cash or whatever you need in here.

If anyone asks, it is for protection and alerting others that you are in danger. 

I printed with 0.12mm layers with a 0.4mm nozzle. You may need to adjust the design depending on the tolerances of your printer.