Hm, cool idea. I wonder if "awning" (is this the correct term?) of the 3233 police car could be customized for this purpose (using two, separating the ends to enlarge in length) ...
... and then sprayed in THW-blue.
Hello Jochen, the technical term is a tilt, although awning is more commonly used and seems always to be used in relation to ships or boats. Lots of people may not understand "tilt" used in this way - I certainly didn't when I first came across it! "Cover" is a possible word, too.
I wondered about the cover from the earlier Playmobil trucks, too, but never having seen one in real life or for sale, I didn't take it further.
I've had some spray paint made up to RAL 5002 at the local branch of Halfords, a motor accessory and cycle chain of shops. My next custom just needs a bit more painting and it will be ready to show. Over here, RAL 5002 is described as ultramarine blue rather than "king's" blue.