My initial reply may be heretical. Start with a length of plastic plumbimg pipe, say 1.5-2 inches ID. Get hold of a guitar tuner.

Hack off a length no longer than the distance of your mouth to the floor when standing. Tune it with the tuner. Now experiment by hacking off small pieces, say half an inch. The tone will get higher the more you hack.

If you prune the length too hard you can always glue a small piece to the end - or even insert a small tube of card or paper - in order to deepen the sound.

Once you get the length right for a particular note, just jot down the details of the length for that tone. So you may wish to end up with a D didg for playing along with an Irish tune. Want to accompany someone singing in C, just clip on your extra length of pipe with a piece of tape and hey presto.

I'm sure that the physicists among us could come up with some calculations as to appropriate lengths for a given diameter of pipe.

Regards John