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9mm moisture resistant Mdf can be quite useful. Hardboard even. You might find it easier than applying the filler more roughly and sanding it back. I would expect it to take 2 or 3 passes gradually sanding it back to shape but I would recommend using a template for the sanding too.
It's pretty easy to trace the outline onto hardboard and cut it out with a box knife. Mdf if you're handy with a jigsaw.
What would I make it out of?