Record through your own outer ears.
The Plug is a blocked-ear binaural microphone. Its microphones sit at the blocked ear-canal entrances and face outward, so your own head, pinnae and conchae shape the sound before it reaches the microphones.
Keep the anatomy that creates the cues.
Because the microphone body sits at the ear entrance rather than occupying much of the concha, the listener's own outer-ear geometry remains substantially unobstructed. The downstream ear-canal transfer is excluded from the recording, while the individual head, pinna and concha cues remain in the captured left and right signals.
Full stereo directly over USB-C.
BOYA BY-K4
The BOYA BY-K4 accepts the Plug's stereo 3.5 mm TRS microphone signal and keeps the left and right microphones as separate USB audio channels.
That gives the Plug a simple, low-cost path to binaural recording on a USB-C smartphone, tablet or computer. The adapter is inexpensive and widely available, including through Amazon.
Do not substitute a normal USB-C headset dongle. Those usually provide only a mono microphone input even though they can look almost identical.
For standalone recording, a portable recorder such as the Tascam DR-05XP is another full-stereo option. See the setup guide for the current connection hierarchy.
Record and calibrate at the same acoustic reference point.
The same Plug can later measure the headphones at essentially the same blocked-ear reference plane used for the recording. For playback of your own binaural recording, that removes the need to define a separate generic headphone target.
The recording itself provides the playback reference.
Conceptually, the headphone calibration becomes the inverse of the measured headphone transfer function at the Plug reference plane. When you remove the Plug and listen normally, your own real ear canals are physically back in the reproduction path.
A clean reference architecture, not a claim of mathematical perfection.
The headphone is measured while the entrance is blocked by the Plug and listened to with the Plug removed. If a headphone changes response significantly with acoustic load, the blocked measurement can differ from normal listening. This is especially relevant for some pressure-sensitive closed-back headphones.
The important advantage is that the separate target-definition problem disappears for your own binaural recordings. The remaining caveat is headphone loading, not uncertainty about where the recording and headphone measurement are referenced.