Many thanks to Dave C., who emailed me a whole passel of articles on pre-Conquest and medieval bone flutes!
Indeed, I jumped the gun a while back. Just because the 40,000-year old flutes were notched and end blown doesn't mean the Anglo-Saxon ones were and, indeed, the archaeological finds seemed to all be fipple flutes. I have to go over the articles in-depth, but some measurements jumped out: a fairly intact, large specimen 19cm long and about 11mm in diameter. That's about a pennywhistle with the last three holes cut off.
Remember the PVC pipes I bought back in March? Finally hauled one of the smaller diameter ones out and drilled two holes in it, near either end. I used a mitre box and saw to try and turn one of the holes into a D-shaped blow hole, then used a small square file to file down the edge. It looks about right.
No fipples survive; beeswax has been suggested as a possibly substance that would conform to the irregular inside of a cleaned bone. I don't have any beeswax, but thanks to having two small boys, I have an abundance of Play-Doh.
Play-Doh does not make a very good fipple, at least not right off the bat. It's too soft. I can plug up the entire end of the flute with it, but when I try to make that tiny channel for air along the top, everything just goes squishy.
Maybe I should make a plug and let it air-dry, then file off a flat spot? It'll shrink as it dries, but a thin layer of fresh 'Doh ought to make up the difference.
In other news: Lyre! Hoo boy.