I tried to tint plexiglass using the soap and water method.  It does work, but the main issue that you'll find is that it's difficult to tint a highly curved surface.  I also found that when I did get the tint on nice a neat, it crinkled as soon as I installed the visor.  I apparently didn't have the visor curved to the same amount as when it's installed.    :facepalm
For the cost of a tinted safety shield, in my case it was under $8 Canadian including the taxes, it's just not worth the headache.    :cheers