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I rebuilt the tail lights on my trailer about 10 years ago. The options at the time were to replace the metal "can" that houses the light bulb, the bulb with an LED one, and the lens. Alternatively, I could remove the can and install an all-in-one sealed light assembly.
I did the latter. I think I bought them from InlandRV.com. The cost was similar to the product you show in your listing. What I got from them was a light that was approximately the right size to fit into the casting, but it had been shaved down, to remove a seam or mounting holes or something. To install them, they were just glued in place using silicone.