I have a piece of 1/4" tempered glass on top of my building table (not building board) It is pretty flat but I decided once to use it as a surface while I sheeted a foam wing. I put the wings in the shucks and put my building board on top of them and weighted it down with a couple hundred or more pounds of weight. I didn't know it at the time but this put a nice even gentle bow into everything. The top was up so all it did was add a little dihedral and it flew just fine. The point here is glass all by itself is indeed flat but strange as it seems it can be bowed without breaking.