I can't see the original image links but i can see the links in Sedi's quote. And I must say, that's a clean looking first whistle. I can envision you making some good stuff.
This is why I pretty much skipped using pvc (not that I should have). I'm in the same boat, all the pvc I've ever bought was at least 3mm thick walls. A possible issue with the duct tape is that as you make it higher, the lip will seem lower. Normally you want the lip to be visible through the windway, either just slightly at the bottom, or some like the feadog, its pretty much in the middle. It can work while not visible, but if you tried to make the windway much more efficient with more tape it would run into issues. Like one case can be when you try to overblow for higher notes, you get no note at all.
That said because your tape fix worked, heres a more permanent version- Make a wooden recorder block. Its much more annoying than just finding a dowel that fits in. But it has the flat shoulders that come up into the windway, so if you can get it right it will be like your tape but much more permanent.
http://www.flute-a-bec.com/bouchongb.html
I've seen another maker mention using polycarbonate tubing before, you may be able to find that in thinner tubes.
My actual recommendation- Another option is if you could find a tube thats too small to fit over your pvc, you could sand the main pvc tube down so that the other will fit. This will make the windway less than the 3mm it is normally. So If your 3/4 pvc tube is 15mm inner, with 3mm walls, its 21mm outer. So if you could find a tube thats 17mm inner, you could sand your pvc so that tube fits over. And you now should have a 1mm windway instead of 3mm. This can be tricky without a lathe to perfectly sand it evenly, but should still be doable. I'd probobly recommend trying this as my best idea. If you want to try this, you'd probobly want to go about it similar to how the guy describes makign the recorder block. Try to measure and draw lines in your tube to mark how far in you want to sand. Then when you get it almost as small as you need, use the tube you are tryign to fit over to try and get it perfect. It will take a bit of practice so you may want to start with normal 'not a whistle' pvc, but once you are good at it I'd expect you could do it easily and consistently, makign it a good idea. Note that I've never done this myself, I've only taking off like 0.5-1mm off tubes to make tuning slides and stuff.
My thoughts on using aluminum tubes instead of pvc, which is what I tried to do before when I didnt want to use pvc - Aluminum tubing is pretty easy to find in either 1mm or 1.5mm walls. The hard part with that is For me they never actually fit together well, so you need to do a decent amount of sanding to make them fit (or they were lose and dont seal in any way, and is useless). Which you'd want at least a belt sander for, or a lathe. And sanding a 1mm tube down so a tube fits over it can make the windway too small. So for these reasons while using aluminum could sound easier, it has its own struggles. So I'd just recommend trying my above suggestion using your 3/4 pvc.
But theres many more solutions and ways to make them than I've just mentioned, so I trust that you will find the best method for how you want to make them. While I've found making whistles to be much more complicated than the drilling holes in tubes and making a lip like I originally thought, that just makes it so when you make somethign your happy with it feels that much cooler.
