John, that's a great idea. I do recall seeing some DIY spectroscope before. (here it is)
The design would obviously have to be a bit different than this example since you'd want to have a reference light source and to project the spectrum onto a film sample. I would think that if you're messing around with spectral sensitizing you'd really want to have some way to qualify what you're doing.
Ron, who made monochromators designed for this kind of thing?
Maybe we should make Bill one so he doesn't have to keep pounding his pud all day...