Nice design but what makes you think its not waterproof as is? Aren't you supposed to close the cover on the wire after you plug it in? I think I see a gasket there? A normal J1772 connector than would have the same issue? Why is that waterproof?

 

The manufacturer states it is not waterproof. There are no internal seals. Closing the cover doesn't make it waterproof.

 

Or perhaps if you take it apart, you could re-assemble with silicon sealant at the joints?

 

That still wouldn't fully waterproof the extension cable plug in its socket, especially rain driven by strong winds.

 

Did you know you can buy magnetic rain covers that seal up against the body of the car & cover the flap as well
This one appears the right sort of size to take the V2L adaptor - Just my opinion on the size - nothing measured

On another note - Do yourself a favour & throw out that extension lead you used - You can see where the plastic is cracking up around the sockets due to age & UV exposure

 

Similar one bought from Amazon. It does offer protection for the V2L adapter but due to the creases in the car bodywork and location of the charger port there are gaps either side. It maybe I just need to play around to find the best orientation for the cover as the magnetic rim is in two pieces.