Whatever you do,stop!!! and download the LLNL paper attached.
If you want to build large lenses and if optical glass is not available to your lens size, very expensive or at foreign company,you can make your exotic Lawrence Livermore Laboratories Laser Fusion experiments grade KDP mono crystal, 320 kilograms of mass in few months,only preparing an saturated liquid,an turntable and heating the mass 40 to 70 celcius degrees.
I prepared a basic intro, google KDP and growing.Growth rate 15 to 25 mm a day. If you can make it fill a mold,you would not be needed to cnc later
There are hundreds of papers at google and you can learn as much as you want.You can order a cut giant cylinderical lens, cone lens, aspheric lens elements with cnc or diamond turning.
A single crystal or monocrystalline solid is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries. The absence of the defects associated with grain boundaries can give monocrystals unique properties, particularly mechanical, optical and electrical, which can also be anisotropic, depending on the type of crystallographic structure. These properties, in addition to making them precious in some gems, are industrially used in technological applications, especially in optics and electronics.
Mustafa Umut Sarac