It turns out that while the open source stethoscope was tested with PVC diaphragms, biaxially-oriented PET (BoPET, Mylar) is what is used professionally. I compared to a “real” diaphragm to the PVC and I can hear the difference in sound quality. The “real” one is 0.2mm thick, so I ordered 7.5mil mylar film (the closest match I can find) to test with.
I note that BoPET would be appropriate to cut with a laser. 
The 7.5mm BoPET measures the same as the professional diaphragm within the precision of my calipers.
Now’s the best excuse to buy a laser!
Were you successful in adding a mic to this stethoscope? I am working on very similar, and intending to embed the mic in the head. What did you do and how well does it work?
Oh, I forgot about this one. At about that time this particular need in my area seemed to vanish, so I quit trying to work on this and focused on other things.
Ok, thanks for the response.