I wouldn't call it the plot of the Cube, more like the setting/world-building.

I didn’t get that part of The Cube, I thought it was just like this huge, sadistic contraption?

The plot is about the people trapped in the Cube trying to figure out their situation and get out.

The construction of the Cube is kind of a backstory, not the main part.

Eagle Eye too, with Shia LaBeouf, although people in that story are constrained into doing specific small tasks, not knowing for whom, why or what is the endgame.

I actually like that plot device.