Sweet idea!
Some concern about the magnets.
I'm no expert on the subject matter, but I've heard quite a number of discussions that the magnets will gather the swarf onto the file and it will be hard to remove - because the file itself will be magnetized.
Though it's hard to argue with a successful outcome. So, if it works long-term for you, it's probably a theoretical concern, not a real-life one. I just know I've heard it mentioned often when talking about magnets and tools. (And marble, music-playing machines.)
IMO, the "plane" carrier will make using the tool to really flatten the base way more consistent, and less likely to add bevel to the edges (unintentionally), or otherwise do unintended damage to the base. Plus it's going to be easier to use than just a plain file, IMO.
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