The front pivot arm of the 11spd FD Di2 is plastic, the, rear driver arm is metal. Bending the cage would have to
take into account the relative fragility of the motor and its connection to the arms, as well as the plastic
front pivot arm. In theory the rivets holding the cage to the arms could be ground off and the cage removed
I suspect a donor cage would have to come from another Di2 FD. Access to a cluster of 10-11spd mechanical
FDs for cage comparison might point to a possible cheaper donor FD but Shimano probably didn't use
similar enough pivot arm designs for this to work. New FD ? $150 or so your best bet.

I believe I have a spare 10 spd Di2 FD but these are not usable directly on 11 spd setups. Have to find it
before I can compare the cages.