the best solution i've found w/ the port cover is to not try in the first place. Have not had problems riding in the rain w/ the plug exposed. Obviously I'm taking chances there, but IMO you're gnashing your teeth at nothing trying to put that thing on.
Wow, it's not that hard to secure!
Even if it was, personally, I wouldn't risk running around in the rain with it dangling open. Sell it while it still works, man, and get something else!