I have looked for internal seatpost cable routing gromment for too long, so here is my take.
It is optimal for use with 4 mm cable width. Standard jagwire L3 cable housing is actually 4.15 mm, so the hole diameter in this design is 4.6 mm.
I've tried to keep the insert hole dimensions for drilling to a minimum. First iteration was too short creating undesirable tension in the housing. I added another 5 mm to the length and the bottom hole is now offset to account for frame material wifth. - width 6 mm, length 25 mm, cable angle 17°.
This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/gN9mVZ5OpwU