Thanks for your suggestions. I had a closer look tonight and there is a beam going across the roof right behind the ceiling light. I pressed on this in a few strategic places. A very subjective assessment says it helped a tiny bit, well maybe not at all...? I will try the heating/shock cooling first with a hair dryer. I have a feeling that such a large flat piece of sheet metal will very easily warp, so maybe it doesn't take that much to bring it back either.
George, I like your idea too, but will have to find some type of suction cup first.
I'll give an update after I've tried the heat/cool stunt.

Kåre