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Exactly what I'm going to try today. Thank you T and WojtekGood stuff H. Just take your time and work carefully and methodically, and it'll turn out OK. If you have a small enough file, the inside edges can be filed, if not, then gently scrape the edges with the scalpel blade. Don't try to cut, as this might remove too much, or you might slip and really make it hard to correct. Sort of carefully shave the edges and it should be fine.
I'd be using my Dremel, then sand for finish