I have a Foil 15 as mentioned above, I wrapped the battery in some bubble wrap w/tape, and dropped it down into the seat tube, also rigged a wire to pull it out if necessary in the tape. Wires aren't an issue anywhere, very simple to put it in the seat tube using the internal Di2 battery. Never had any rattling or issues with it. Then I left the bolt holes with nothing in those two for the intended external/below bottle holder external battery on the DT. Just used a rubber frame plug to plug the open hole below that.
New Scott Foil has a really clever integrated bar/stem that is completely adjustable, hides all the cables, has a garmin out front mount AND, you can hide your Di2 junction box in it as well.
You can see on my frame Foil 15 the two holes for the external, then the little black grommet covering the open hole intended for the wire below that/above the big ring chain in the photos. Battery is inside the ST of course as mentioned, and the FD wire comes out below the ST water bottle holder.
Check out this link and down the page for photos of the clever bar/holes/mount/Di2 spot in this link on the new 2016 Foil.http://cyclingtips.com.au/2015/06/first ... cott-foil/