I have a not-terribly-expensive whistle that is far too easy to overblow. Is there a way to increase the backpressure without cutting down on the volume? The mouthpiece is delrin, I think, or some other plastic.
Non-permanent would be best, but I’ll entertain all possibilites.
Thx,
Tyg
Sure, graft an Overton head onto it, that’ll fix her. 
Loren
Someone (Vomit?) mentioned wedging a couple toothpicks in each side of the windway to narrow it. Of course, you could probably trim little bits of wood to produce different widths. If friction won’t keep them in place, white glue might hold them without being permanent.
Hello Ty
The problem is probably the shallow angle of the labium ramp or an oversized bore diameter/length ratio.
I suggest adding a “ring” of Blu-tack or similar putty to the hole at the end of the bore to reduce it’s size. This will flatten the bell note, but you can compensate with more breath.
Another option would be to increase the “angle” of the labium ramp with the same putty, or maybe cut up an old clarinet or sax reed and add it to the ramp height.
If these don’t work for you, there are other “options” we can try.
Hope this helps and good luck!