glass for developing presensitized pcbs
Glass can most certainly bend.

A standard method to ensure good glass to art to board (or film) contact is to use a vacuum pump (might be able to improvise sufficiently with a vacuum cleaner) to get them all "pressed" together nicely. This is common for contact prints on a large scale and for PCB exposure.

Super-thick glass is not required. Typically there is a gasket larger than the largest working area needed, a glass a bit larger than the gasket, some layers of perforated material to aid in even vacuum pressure.