Waterproof windowsills

We've added a room to our house with lots of windows. It's time to build the windowsills, and my husband is planning to build them of wood as that's what he's used to. I've seen new homes with acrylic or some other type of windowsill and wonder if those materials are available to diy-ers, what it's called, where I would find it, if it is hard to work with. Alternatively, is there a good way to treat / coat painted wood windowsills to make them impervious to water etc from condensation, flowerpots, ...
Thanks -

 

There are various synthetic woods, normally used outside that could be used if you were going to paint them. Aztec comes to mind, and I'm sure there are others. Most can be cut, shaped, treated like regular wood.

 

Sills and other trim sized boards are available in PVC. Usually just seen it called PVC Trim