A friend gave me a bamboo flute in G, and I’m thinking that it would make a neat whistle. What would be the best way to convert the flute to an end-blown whistle-type instrument? Has anyone attempted this type of hack before?
I think the best way to convert a flute into a whistle is to learn to play the flute. 
(and in any case I don’t trust myself telling other people how to work bamboo.)
A while ago someone mentioned in a thread about whistle collections that they had a piccolo flute that was converted to a whistle. I meant to ask for more information about that instrument, but forgot and now I can’t find the thread, so I don’t know who it was.
Owen
Never tried it myself but it can be done. Several whistle makers offer extra heads to convert their whistles to flutes or fifes. I’d measure the diameter of the flute then find a whistle head fits and see what you get. You’ll probably need to wrap the bamboo tube with teflon tape or something similar to get the new mouthpiece to fit. I’m guessing you’ll destroy the flute as a flute since you’ll have to cut off the flute mouthpiece to fit the new head. If the flute is a toy, it might be worth experimenting with; if it’s a good flute, maybe not.
It can be done. I can think of two ways to approach it. There are likely others as well. In the first case you could cut the windway into the tube from the top of the head-end to the blow hole. You would bevel the edge of the blow hole to make the labium, plug the tube and wrap the head with the material of your choice to make the outter mouthpiece. This would effect the tuning making everything a bit sharp though. The second method would involve making a whole whistle mouthpiece and fitting it to the flute body. If you haven’t made whistles prior to this, I would caution you not to do this for fear of ruining the flute. I’d tend to agree with The SpoonMan, learn to play the flute. Your results may be better and come quicker.
Feadoggie
trade the flute for a whistle. less work.
It’s doable but you need to strictly maintain the bore diameter between the two pieces. Otherwise, the second octave will play off pitch.