Shimano E-Tube Ultegra Di2 Short Battery Mount SM-BMR2-S1 Black | eBay
Zipp SL Speed DI2 Battery Mount - Clutch Bike Shop | Atlanta, GA
PRO Di2 Battery Holder Seatpost Di2 Built in Battery Mounting Bracket For Seat Rail Mounting 27.2/28.6/30.9/31.6|Bicycle Seat Post| - AliExpress
Fouriers DI2 Junction Battery Mount Holder For S H I M A N O Di2 EW90A EW90B Junction Extention Adaptor 6 Degree|batteri|control hvaccontrol airplane - AliExpress
Installing internal DI2 battery into aero frame w/integrated seat post - Bike Forums