Have you queried the Cannondale dealer about plugs? I became aware that several major bike makers
supply different plug/grommets to match Di2 wires versus cable housing, as typically the opening is a housing stop
but the Di2 connector is as large as the housing and must pass through. Varies, no specifics. Shimano
sells grommets for Di2 frame installs FWIW, whether they would fit or not?? On my Mekk frame the opening
is oversized and filled with a 2x1.5cm plastic bit that closes the opening and is held in by a screw. It was
clearly a cable housing stop and could have been drilled out for Di2 but it was easy to fabricate a replacement.