Shimano's remote aero bar / TT shifter SW-9071 allows you to shift your Di2 rear derailleur while using the aero bar / time trial extension. A pair of them weighs 124g.

These switches will fit extensions with an inner diameter of 19.0 - 22.5 mm

Synchronized Shift only

If you don't use a front derailleur on your time trial bike feel free to ignore this, but for those of you that do - these shifters can only be used with Synchronized Shift. They only have one button each, so you generally set one up to shift the RD to an easier gear and the other to shift to a harder gear.

Using Synchro Shift would make the bike shift the front derailleur automatically, so not being able to shift the FD using a button or shifter is no problem.