You're not wrong Dave, but the black box isn't the only or the best option. You can also set it to make a "full shadow". This uses the full original mesh but replaces the textures with its own built-in black texture (ie. you don't need to provide any images or texture.txt files to go with the shadow).
While you're busy setting the Options, it's also a good idea to set the X,Y,Z scale to a little less than 100% of the main mesh outer dimensions. This is so the black shadow mesh will never poke outside the main mesh. I usually have X,Y,Z set to 0.95 (=95%).