People who follow my display board projects know that I usually design them to be cascaded for more digits. They are connected in series using either Dupont pin sockets and pin headers, or just soldered jumpers as in the photo above. Neither is satisfactory as there is still flexure. I have considered using small aluminium straps and screws to hold the boards together stiffly using the corner holes. But I would have to look for aluminium strip of the right width and hacksaw small lengths off.

Today I tried another idea, using reusable paper clips that were designed to hold multiple sheets of a document together. They are attached to a corner of the document stack with a special tool. It's a solution between standard paper clips and fold back paper clips.

It works and I would also need to put some hot glue on the clips to prevent them from sliding off. However in future I will also need to design enough clearance so that the clip doesn't short PCB connections as the one on top in the picture almost does.