If it is real, I would assume it was made from Plexiglas, the same material used to make the canopies for the P51 Mustang and other US fighter planes late in WW2.
Yes they were made as an after market item.
I do not know who made them, but a gentleman named Barrett MacGregor owned one. I believe that he lived in northern Wisconsin or the UP of Michigan.
I saw the car once in a parade that I stumbled across in Iron Mountain MI back in 1979. I was on my way to the Eastern National Meet in Dearborn MI and took the long way around.
I also think that there was a story about the plastic top in the V-8 TIMES at one time.
Check out this issue of the V-8 TIMES