That looks like the eBay special "Lucas Style Taillight" that I know some of us have purchased back in the day. I have that installed on my bike (need to dig up a photo of that). What I did is take a piece of rubber tubing and split it down the middle (length) to form a sort of "gasket" between the fender and the taillight housing. I also drilled the sides of the taillight housing to install some small aftermarket turn signals.
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