Tommy Dion also makes a very nice low D from pvc in either one part, two part tunable, or three part tunable.
I believe there is a picture of a two part at this link. CP Woodwhistle pvc copper prototype makers tip moving holes - Whistle - Chiff & Fipple
And a link to a picture with review of a three part tunable bought at a dulcimer fest in Palestine Texas.
A link to a sound clip here. http://praisewhistlers.org/forum/viewtopic.php?f=25&t=5704
I like the three part tunable for backpacking, and in the kayak. 
The one part fits inside of a mountain dulcimer case with plenty of room for the dulcimer.