...UNDER DEVELOPMENT - to be expanded in time...
Short Synopsis :
If one has a model with a hard, smooth surface, which tends to rule out making a mold using a silicone caulking/mineral spirits slurry (see Silicone Caulking Mold - I), it is still possible to make a relatively serviceable mold.
Coat the model with some type of mold release (e.g. petroleum jelly), then simply use a caulking gun to give the entire model one "continuous bead" of caulking. It can take awhile, and one does have to be careful to avoid trapping air bubbles, but one can get in the end a mold which works quite well.