A Y cable may be the most convenient because a one-piece connector may be too bulky to connect. You can of course avoid this problem, if you need to, by putting a large capacitance in series. You may not have this problem with yours. If you do, it might be worth making up the connector below. Buy a cable with a stereo mini-phono jack on each end and cut it in half.
Lapel microphones are very good value for money.
Flat response, little variability from one to another. Jack probably fits the sound card in your computers. Plastic conduit or sewage pipe makes great acoustical waveguides , and you can buy closed ends, corners etc ready made and relatively cheaply. For several dollars you can buy the diode plus power supply. Just add a battery.