Nice camera. Congratulations.
I'm building a 16x20 as well and it is almost finished (a little paint and velvet and it is ready).
Good luck with your pictures.
I'm curious. As it isn't in the build photos, did you attach it to the back of the lens or to the front like a typical filter? Also, where did you find a negative diopter lens of such a diameter?
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.