Sculpting Blender advice - joints and hollows?

Don't know if you still need help, but I create hollows by first duplicating my object. I then use solidify, it will look awful on complex objects but it doesn't matter, as it's a copy. Make it about the thickness you want, now apply the modifier. I then go into edit mode, and select a face either on the inside or outside, doesn't matter, and press ctrl + L. If just the inside or outside gets selected, then press p and separate by selection. If the entire model gets selected, then you will need to go into sculpt mode and smooth it as much as you can, and go into edit mode and try again at separation. Ball and sockets is difficult to explain, especially as there different styles of joints, but you do need booleans. I want to eventually make an in depth blog post about joints, so I'll try and post a link here when I am able to do that.

Hope this helps.