This week, I made a 2.8 shakuhachi from a length of PVC pipe and a pipe coupling. I spent about 500 yen (four pounds-ish) in total, not including tools which I've accumulated over the years, which bought enough bits to make three or four flutes.
I think a 2.8 shakuhachi is about the longest standard size.
I won't go into too much detail on how I made the flute here, suffice to say I obtained most of my information from this site - an excellent and entertaining resource for cheapskate instrument builders like myself.
Very briefly, I cut the pipe roughly to length, attached the coupling and cut/rasped/filed the mouthpiece, and checked that I could blow a ロ (RO - the lowest note on a shakuhachi). Then I measured where the holes should be, and applied stickers which I then moved around the pipe (keeping the same distances from the end) until I could comfortably reach all of them. The length of the flute means that it's a bit of a stretch. After that, I drilled and filed the holes until it was roughly in tune.
I also inserted an experimental "Choke Point" - a smaller ring of PVC pipe, about a centimetre long. This is supposed to imitate the effect of the tapered bore of a real shakuhachi, and I wanted to see if it would help with the tuning - with a perfectly cylindrical bore, an end-blown flute is flat when overblowing to the second octave. I pushed it up and down the main tube using a dowel, until it was roughly in the right place (about 9cm from the bottom of the flute) - I used a lot of guesswork here.
Playing the flute with a tuner shows that most of the notes aren't more than 10 cents out. The bottom RO seems quite flat, which could be a result of my "choke", but it also could be my technique as I'm finding that note quite difficult to play. Longer shakuhachis require a great deal of control when playing the deep notes.
Here's a short demonstration. I'm playing the honkyoku "Honshirabe", which I've been studying recently with my teacher. Apologies for the mistakes, huffing and puffing, squeaking etc...