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