GLFaria":3ip3xqw6 said:
I believe an 8mm thick, not tempered (to avoid deformetions), float glass plate ...
I would have gone for a 6 mm thick plate.
If you float a puddle of molten glass on molten tin, it will naturally form a layer about 6 mm thick. Thicknesses other than 6mm have to use some technique to thicken or thin the glass [0], and thus risk distortion from the natural surfaces - which, subject to the local gravitational and magnetic conditions, aught to be a fraction of the surface of a sphere same radius as the earth.
Unless there's something specific about the 8mm thick glass, my instinct is to go for the 6mm, in the hope that the natural flatness is preserved in manufacture; and accept that it might need backing with something for mechanical strength (MDF would be the perfect material for that - flat, slightly springy, and cheap.)
[0] Specifically, pulling or compressing the glass with rollers when semi-solid, as it's lifted off the molten tin.