Erik Kostson
Ansys Employee
Since the plates are not important just neglect the stresses there.
Just scope a remote displacement on the top and bottom plate and on their top and bottom surfaces respectively and set behaviour to rigid and this will make the plates very rigid - again just neglect the stress there.
THat is the easiest way.

The other way is to create the rigid plates in WB, and in a mechanical model system (system E - set them to rigid there) that is combined with the external model (system A below the abaqus model) into a static system (system F). In system F (static) you can then suppress the plates of the original abaqus external model import.

All the best