The giveaway here is the gallery it's been placed in; Manipulation. If the rest of the shape were included, we'd see the neck, and maybe part of the base, of an old desk lamp, possibly dating to the 50's or 60's. I saw lamps like this when I was a kid. A nice play on old Sci-Fi B movies but I think I might have chosen a more definitive background. Whatever the case, you did a fine job of masking and good luck in the contest.
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A good manipulation and post work on this image. The dark diagonal in the upper corner could be removed from the scene. Some blur could be added to the UFO to soften the edges so it blends in more with the background. Best of luck in the contest.
-Nik