I used an already existing whistle and modified it to have finger grips.  I didn't want to have to spend $15-20 for a whistle when I could make one. I couldn't find one like this on printables or thingiverse, so that is why I made this. No supports needed, but make sure to get a small screwdriver to pop the ball inside loose once printed. 

Update: I tested it and it is slightly higher pitched than the whistles that you can buy, but it should still work fine.