Basically just a doubled version of the backing plate uploaded by acbaruth. It holds the two breadboards together more securely while still having a minimal footprint. It mostly works with my myriad 830 tie breadboards, but many have very slightly different spacing for the screws, but it still works great.

Updated:

I didn't account for the size of my screw's heads when I made the first version and were about 0.55mm too long. The second version should address that. The screws I'm using are (the closest I could find at my local hardware store that fit) are self-tapping #2 x 1/4'', pan head screws for wood/metal.