You could make a gumpaste one with the template and support it with small pieces of foam or paper towel until the gumpaste hardened. The only problem I would foresee is if it was to be quite a large hat then it might be just too floppy to support well.
Alternatively, carve some cake so that it's a kind of wedge shape but rounded a bit at the edges. This would give you the wedge shape of the main part of the hat (see webpage below).
Either crumb coat it and cover it with a layer of fondant to match your main cake, or cover it in buttercream to match your main cake. Then lay on three strips of white fondant to look like the folded section. Then cut a rectangle to make the piece of the hat with the stripe that goes around the sides and front.
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