I've had success with two similar approaches...
the first was to have the stethescope place in the impression material to
give the manufacturer the option of modifying the face plate..generally
with an indentation where the ear tips are...
the second was to build a small annulus around the *microphone* opening
over which the port of the ear tips fit...sometimes had to fiddle with
different types of tips
Joseph Danto, Ph.D.
Professor & Program Director
Speech Pathology Audiology
212 340 7806
201 567 8408
201 567 7551 (facs)