What material is this?

[briwire]

I've seen fittings like window ledges made out of a rigid plastic that seems to be made with a foam-type interior, or maybe I imagined it!

Any idea what it is and where I can get some from. I want to fill in a gap at the end of a bath with a horizontal piece. An architrave shape would be OK, about 3 inches wide. I can get MDF or normal timber but thought that the one above would be more resistant to water.

 

[crank39]

Its pvc trim, commonly used in the window trade...

http://www.casupply.co.uk/acatalog/fascia_soffits_cladding_trims.html

Scroll down to Deeplas 6mm thick architrave