Back in the mid 70's, I made (and briefly used) a 6" aluminum telescope mirror. The aluminum mirror blanks and processing instructions came from a
company called Innovative Optical Materiel. My mirror (a standard f8 design) worked well, although the natural microscopic roughness of even polished
aluminum produced a slight halo around bright objects.
Innovative Optical Materiel is long out of business. The owner, Tom Settimi expanded the original company into Sky Scientific. Among their products is a range of inexpensive aluminum mirror blanks.
If you decide to make an aluminum mirror, I'd like to hear about your experiences. The blanks from Sky Scientific are ready to use and they include updated grinding and processing instructions. For a larger mirror (6" - 8") just about any aluminum should work for the blank, although T6061 is probably the easiest to obtain.
Sagitta at any radius of a spherical mirror:
S = ROC - SQRT (ROC^2 - r^2)
Sr = SQRT (ROC^2 - y^2) - SQRT (ROC^2 - r^2)
where ROC = radius of curvature
r = radius of mirror
y = radius at which sagitta is measured.