> I don't have the tools or knowledge required to sand this down to a perfect mirror finish
I did a resin cast that had a similar "lip" at the edge (I turned a LGA CPU socket into a coaster), and I was able to sand it down fairly easily using regular sandpaper of progressively higher grit, producing a more or less perfect cuboid. The flatness was achieved by taping the sandpaper to a flat surface, and moving the part.
It's been a while, I can't remember if I used some kind of polishing compound at the end or whether the sandpaper alone was enough for a good finish.
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