back of lens is best--it works only to increase the focal length, and, in turn, effective coverage of the lens. Of course it degrades quality, but when you have huge plate to cover, it works. You can get negative diopters from optician or trial lens sets these days. Maybe surplus shed or the like, optical supply houses.
The teleconverter lenses work on the same principle. They are a negative diopter, diverging lens, that increases focal length. Since the angle of coverage remains the same, the extra distance between the lens and the film plane due to the increased focal length, makes the coverage circle bigger.