When Doug Tipple first sent me one of his flutes to do his YouTube demo clips for him last year, I was frustrated by the small embouchure cut and set about making an experimental flute head based on my own former experiments with GB spec electrical conduit tube. We cannot get anything closely resembling the US schedule 40 plumbing tube Doug uses in Europe, though. I wanted my head to fit into Doug’s flute body, so I took a suitable length of my black PVC conduit in the nearest equivalent size (25mm OD, c 21mm ID) and (lacking any relevant machine tools) hand filed one end of it down to make a tenon until it was getting near the dimensions of Doug’s body socket/tuning slide. I then used 2 or 3 succesively finer grades of sandpaper to rub it down until it did fit - it isn’t totally even nor has a uniform surface, but it fits airtight and does the trick. It didn’t even take horribly long, though I wouldn’t want to do it regularly! I was just itching to do something that it was a necessary step towards, though I was actually more successful than I anticipated. One could probably rig up some kind of adapter for a drill chuck to hold the tube and spin it to simulate a lathe, at least for the sanding phase, though health & safety considerations are moot on that!
Anyway, the upshot was I made a workable one-off male tenon slide to fit into Doug’s flute body, made the piece of tube up into a flute head with a larger embouchure cut and found out with it what I wanted to know - which led on to further experiments (stalled due to time pressures!) in collaboration with Doug on bits of tube he sent me… Meantime, he has of course moved to a larger embouchure cut himself. Relevant point being, you could probably (with a little patience and a good eye) do much the same as I did with a hand file and sandpaper to a piece of the Sched 40 tube and turn it into a whistle head… at least as a prototype. If you want to do multiples, though, you need either to get yourself a lathe of some kind or go the insert-a-metal-joiner-for-a-tuning-slide route. The best (simple DIY) solution for the latter method would be to find some brass tubing with the same ID as the Sched 40 and then warm up the PVC and expand it over the metal tube; glue it in one side and grease the other for your slide (not applicable if using your Tipple flute body and don’t wish it to be damaged/unusable with its original flute head!).
Good luck!