I am needing a lot of setback at the moment, mainly due to using an ISM saddle.
While idly browsing eBay I saw one with 40mm, which is pretty rare. It wasn't expensive so I bought one. It looks good:
Complication is that it has an internal cross-piece...
...which is where I want my Di2 battery to go.
This bit is pretty strong and, while I don't know if it is structural, it would take some cutting out.
I would be having the seat post a long way in, such that the bottom 15 cm or so would not be needed, so I could just cut it down and stick my battery underneath it. Or I could try to take bottom 10cm of the the bit out cleanly, with hacksaw and dremel, etc, to make a space for battery.
Any thoughts before I give it a shot?