the glass is only flat if the underlayment is flat. I used to make big aquariums and 3/8 and 1/2 inch glass will really bow out ona long run. I made on with 1/4 glass that was about three feet long and 10" highwhen filled, there was about a 1/2" bow on both front and back. and just on the top as the bottom was glued to the bottom.

That said though, our shower doors are now over 20 years old and when Ireplace them. the old glass is going on the bench top. If I can get it flat to begin with and it doesn't wilt like plant without water.

Don