Any gaps between the crucibles will have the melted liquid metal spilling out.

So there should be an indentation on the receiving end of the crucible, and protruding end from another in its edges.

How to close those gaps is by this:

Between the cubes should be an rotating valve mechanism.

I could say self-motorized, but that would make things expensive.

Instead, have a machine following the tracks from the inside to rotate a gear in which to move the cubes together. There can be rubber lining between the cubes for better sealing.

When the cubes are being interlocked by a pattern I designed (T pattern). (It can also be other patterns)

There should be a certain gap between these interlocking points as well. It merely needs to descend and be in place. These gaps will be used with the valve in order to close the cubes and crucibles together.

Forming a complete lock.

Then there you have no liquid metal that will spill out.

Welding comes AFTER the valve system locks in the cubes. So then XYZ axis of the cubes must be designed well enough.