I think drilling a small drain hole into the plastic cover is a good idea. Additional sealing of all communication connectors (lower battery module, sevcon controller, motor), which are directly exposed to splashing or sprayed water, is also very helpful to prevent water leakage inside.
In my opinion, only liquid from the engine cooling circuit penetrates into the space under the side plastic cover of the engine power cables. Although the rubber grommet of the temperature sensor cable and the motor rotation sensor is not perfectly watertight, it is very unlikely that more water will penetrate through it from the outside. Nevertheless, additional sealing of this rubber grommet may be useful.
As I understand it, the coolant penetrates through the rubber o-ring, which is located in the groove of the outer metal cover of the engine and is pressed by the cover screws onto the engine body. After the leaking, the liquid drains into the bottom of the sealed plastic lid, where it settles and does not flow out. Due to the consequent increase in engine temperature while driving, this liquid evaporates into the entire space and then corrodes all metal parts around it (rotation sensor cover, power cable contacts, etc.).
I don't know exactly what causes the coolant leaking. Either the material of the O-ring ages (hardens) after a certain time or its structure is chemically disturbed by the aged coolant. Another possibility may be a situation where, due to the extreme temperature of the coolant, its pressure also increases excessively and the o-ring allows a part of the coolant to pass through.
