Missing orange cover on marker light; Water leakage concerns

That looks like a normal marker light. I would look at an auto parts store or one that is rv related. I know when I was in Canada and needed a light for my tow dolly I came across I believe they are call Canada Tire. That store had everything.

As for making a leak proof seal. A small bead of silicone or even plumbers putty would take care of it.