You probably want to add some mechanical stiffeners to your board. It may be too late for that, however, since that is usually factored into the PCB design (unless you want to design a new PCB and mounting fixture).
The stiffener would be a rigid piece of metal or other material that runs along the length of your PCB (or sometimes across the center of it) with fasteners to hold the board tightly against it. It can also provide the mounting interface to your optics and/or thermal relief (heat sink).
#PCB Assembly #PCB Design
