I got a children's slide whistle and did a thing that turned it into quite an amazing instrument.

Inside there's this slidy rubber (I guess) piston thing, with a wire that pokes out the end. It's almost impossible to gauge where to slide it to, to get a specific note when you want it.

I lengthened the wire by bending the loop at the end into a U instead of an O, glueing a piece of brass tubing on the end, and a new wire into that, with a new loop bent into the end of that. So the wire now turns and comes back along the barrel. Here, I'll try to draw a diagram:
                           __________
   ___________[__________
C________________O

Now there's a loop that travels along the side of the whistle's barrel. It's easy to snap some rubber bands around the barrel and position them as indexes for the fingers.

Then I went and put it somewhere in my house and I can't find it.