Shimano Di2 Battery Holder for 27.2 - 28.6mm Seatposts

PRO Di2 Battery Holder for 30.9 - 31.6mm SeatPost - Bikebug

Available in;, - 27.2 / 28.6mm (for seat post ID 20.0 - 23.0 mm), - 30.9 / 31.6mm (for seat post ID 25.4 - 28.0 mm)

M:Part Di2 Seat Post Battery Mount

Pro Di2 Battery Holder for Seat Post — Bike Stop

PRO Seatpost Di2 Battery Holder 022255476096 – Pit Crew Cycles

Texas Cyclesport Batteries & Electric Parts

PRO Di2 Battery Holder Seatpost Di2 Built-in Battery Mounting

Seatposts Inspiral Cycles

Shimano Di2 Battery holder seatpost For 27.2 - 28.6mm seatpost

Shimano PRO Di2 Battery Holder Seatpost Di2 Built-in Battery