Any experience with low-cost but quality PVC whistles?

[brewerpaul]

Yes, it is flat.. but it is in tune with itself. My plan was to do something like you suggested, but I still concluded I couldn't do it. I spent my early youth trying to learn to build and create things with my hands, until I realized that I'm just not able to do it well, to put it mildly. That's why I turned my creativity into programming instead, and my hands *do* work well on a keyboard. The only kind of work I'm able to do with my hands is some very specific maintenance of my guitars.

If you can find the correct size tubing coupler, this job is super easy. All you need is a hacksaw and some sandpaper. If you're worried about making that cut buy a short length of the same pipe and practice a few cuts.
If you're only playing by yourself, forget about it altogether.You only need to be in "correct" pitch if you're playing with others.

[Tor]

I play with others often, so yes.. I need it to be at concert pitch. But the thing is - I can hear if a familiar type of instrument is outside concert pitch even if I play alone, if it's more than a few cents (I don't really need a tuner to tune my guitar either - I hear the A in my head, so I tune the A string and then I can do the other strings from that one). It doesn't bother me - it's not like as with those absolute pitch folks where it's apparently like hearing bad intonation or worse - but I notice it (and that's why I quickly checked the whistle after I got it and tried playing it).
Maybe I'll try to hack the whistle one day, but I now have three good D whistles which cover my needs, so it'll probably stay in the drawer. I would have to be confident I could really do it before I would try.