If you often work with multiple PCBs at the same time, this tray will help you keep everything organized.
 

It was designed specifically for storing and transporting prototype boards in a clean and convenient way. The tray fits standard 1.6 mm thick PCBs and features a 10 × 10 slot layout. This allows you to store up to 10 boards or more, depending on their individual dimensions.

Overall tray size is 193 × 193 mm.
Multiple trays can be stacked to save workspace and keep projects neatly arranged.

I've also designed a smaller version of this PCB tray here PCB holder