Shouldn't it be quite cheap to determine the frequency using a prism, using the angle of refraction? I think you should be able to use known missing frequencies in sunlight to calibrate your set up. Back in a minute going for duckduckgo
So it looks like you need a bit more than a prism to see Fraunhofer lines, but there seem to be online description of how to do it with a with a prism and a CD, maybe you don't even need a prism, there seem to be 'make your own spectroscope' tutorials than mention building one with cereal box, and maybe a lens of some kind
