I have a 12 fret "Concert" OM. It's structurally nearly identical to your guitar.
My favorite capo on it is definitely the G7th Heritage capo. Expensive, yes, but I've never had anything but flawless performance from it.
The Thalia has a 20" radius insert available. My Thalia worked quite well--and is easily the most beautiful capo I own--but it's also pretty big. I grip the neck at an unusual angle and my hand keeps pushing it over. But it sounds great and you probably won't have the same problem.
The best <$30 capo I've used was a Paige Clik for classical guitars. The flat radius on that capo seems to work pretty well with 20".
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