Depends on finish required , if painting mdf is ideal and cheapest. If you want polished finish then hardwood which is also more stable than softwood but most expensive option.
If using MDF it needs to be the MR (moisture resistant) variety. This is often (although not always) green in colour. Many timber merchants carry pre-primed bull nose window boards in a variety of widths. These are generally 22 to 25mm thick
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