I had to replace my on my 2.3-16 recently. It was a plastic piece. My fine local dealer did not sell just the top, instead they sold the whole kit with the thermostat as well for $300+. Not sure if it was a plastic or metal for the kit. I ended up getting one at the wrecking yard for $10. I do recall on the 190E 2.3 I had, it was a metal housing lid.
Just some useless info for you in case you were going to replace one.