I don't know if they are still using the
Marinco brand inlet on the trailer, but if so, give them a call. I had my cover latch go bad so it floated in the wind, called and asked about a replacement & they sent me an entire inlet for free. It was a bit difficult to replace, but maybe they would be willing to send just a cover.
One problem with just replacing the cover - it doesn't appear that there is access to the hinge pin. It might just snap into the body, but if not, you would need to find away to attach the new cover.