Purchased Di2 Battery Holder from Vanilla Bikes to go with my Ultegra Di2 upgrade kit... fantastic product supplied by Vanilla Bikes
used with a 27mm FSA SLK carbon seat tube and grips well. Make sure you set it where you want it when putting in the battery, as it doesn't want to come out easily when its in.
I fitted this in to an Easton carbon post and it seems to fit quite well, snug but not too tight. Also added a nylon tiewrap to the reduced portion to help keep it in place as I am using this on a cross bike and is going to be subject to a lot of shock. Not used in anger yet but will report back if I have any issues.
This was fitted by the local bike shop to a 27.2 3T seatpin.This failed & was ruined & a substitute used.Not suitable for other makes therefore!