The triangular opening just behind the battery pack was first noticed in 2014 and discussed by i3 owners. As much work as BMW put into lowering the i3's aerodynamic drag, it was difficult to imagine why BMW would leave this opening, especially in a BEV, if it didn't serve a purpose and if it really increased aerodynamic drag significantly. No one could explain the purpose of this opening (transaxle cooling?), and as far as I know, no one covered this opening and reported the results.
if you decide to cover this opening, please report whether aerodynamic drag is reduced and if overheating of the transaxle occurs in hot weather.
2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE