Assuming that is a violin for this small size it would be nice to simply use the tooling board materials which can then be sealed.

A foam surface would need at least some light fiberglass and a suitable paint to get your smooth pattern ready for moulding. If you try to paint directly on to a foam it can be difficult to sand as the the foam behind is soft and easily gouged etc when doing this hand sanding. You could also use a suitable timber for CNC machining like Jelutong or even just MDF boards slabbed to be thick enough. 

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