Gypsum Hopper Wagon
Here are files for a Diagram 1/168 (PG001A) Gypsum Hopper Wagon drawn full size so scale to suit your model.
There were only 39 of these wagons built in 1970 to take gypsum to the Northfleet Cement works in Kent. They were designed to work on the Merry Go Round (MGR) loading and unloading technique and look like a smaller version of the widespread Coal MGR wagons.
I am not aware of any existing models of this curious wagon being available in any scale, so there are no transfers available so far.
Enjoy
Jonny