Thanks for the answers to my questions Kirk.
So if we imagine a simple "shoe-box" spectrosensitometer, it might look like this.
Light source (let's imagine a simple tungsten flash-light bulb for now) -> far enough away or with collimating lens -> diffraction grating -> focusing lens -> film plane.
The light needs to hit the grating dead on, and then we need to focus that image sharply onto the focal plane. Any idea what kind of lenses we'd need, and approximate focal lengths? There are some acrylic lens sets that you can get, or we might be able to order some more appropriate lenses, both from Edmund.
It seems one of the lens kits would be great for this; they have all the different types of lenses for abotu $15.