[Yuri]

Here goes. A photo essay in making files. The example is small and flat. You can make it smaller, bigger, curved one way, curved the other way, whatever. Cost you the price of a chopstick and some double-sided tape and sandpaper.
MUCH handier than forever cleaning the gunk out of steel files that you paid far too much for in the first place.

First shot is the stick shaped to your requirements with the double sided tape attached, but not peeled.
Second shot. Same withsandpaper stuck on.
Third. The sandpaper cut off on two sides, showing the star tool for the job.
Fourth. Hey presto! A file just the way I need it.

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[Mibeck]

genius! and you can shape it to contours and stuff!
i think you got a better mousetrap!

[Yuri]

It is genius, but that's not me, I have to admit. Some (including me) woodwind instrument makers use the idea to shape windways. (that's the narrow channel you blow down in whistles and suchlike.) No idea who invented it.
I just read the posts on cleaning needle files, and, well, didn't think that sounds like much fun.