This is a variation of “8 mm Offset, Trio Command Strip Mount - DS Part A” by KeepMaking.

I have modified it to use two 30x3mm magnets instead of a Command Strip.

You will want to use the matching “Trio Snap Connector - DS Part B” to securely mount these to your Multiboard setup.

The setup shown in my pictures consists of four 8x8 tiles, with one of these magnetic mounts centered on each edge. As you can see, they are fairly strong and support a decent amount of weight. Additional magnetic mounts can be added to further increase the support if needed.