... and for non-English members :
a penny weighs 3.56 grams and has a diameter of 20.32 mm...
...that's 0.1256 ounce and 0.8 inch

A Euro 10 cents coin is about the same diameter (19,75 mm) but a bit heavier (4.10 grams).

I guess it's OK to use a 10 cents coin.... a bit more pressure inside the stove maybe...
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Alain