A diffuse material based on data from http://e-conservation.org/issue-2/36-fors-spectral-database
I'm releasing it under public domain so you can use it or modify it however you like.

The image is small and the graph axis aren't perfectly aligned to pixels, which makes it hard to get accurate data, but I've done the best I can. I also only measured the image every 25nm along the wavelength axis, and a higher accuracy could be achieved by taking more samples. The data I used is shown below. Pixel location measurements are after some cropping to better align image axis.
px at 400 nm 26
px at 800 nm 234
px at 100%: 268

Horizontal pixels, Wavelength, Pixels, Reflectance (out of 1)
26, 400, 12, 0.044776119402985
39, 425, 23.5, 0.087686567164179
52, 450, 19, 0.07089552238806
65, 475, 13, 0.048507462686567
78, 500, 10.5, 0.039179104477612
91, 525, 8, 0.029850746268657
104, 550, 6, 0.022388059701493
117, 575, 4, 0.014925373134328
130, 600, 3.5, 0.013059701492537
143, 625, 2, 0.007462686567164
156, 650, 2, 0.007462686567164
169, 675, 2, 0.007462686567164
182, 700, 4, 0.014925373134328
195, 725, 12, 0.044776119402985
208, 750, 58, 0.216417910447761
221, 775, 130, 0.485074626865672
234, 800, 166, 0.619402985074627