Making the ground of a diorama is quite simple. The plaster comes most easily to mind but it’s most often a bad choice. The plaster is easily breakable and doesn’t stick to anything, especially wood. Besides, the plaster dries very quickly, you have less than 5 minutes to work with. I usually prefer to use filling coat. Of course filling coasts are made of plaster but also adhesives and retarders that give time to work. The lunar ground of the Eagle Transporter has been made this way.