fixture for moving of a part paralell itself

Hello! I simulate a bending sheet steel with a bending angle 90°.
In the assembly there are two of it, and a rigid part which two ends stand on horizontal faces of sheets. Sheets are fixed to rigid part. The rigid part gives a mass load to sheets.

So horizontal sheet faces can move vertical, paralell with the gravity, but their horizontal surface stands always paralell.

How can i fix the horizontal faces, so that they stand always horizontal and stands always a plane and not to be bended. But they can move down paralell to themselves, in vertical direction (in the direction of gravity). Which fixture shall i use for these horizontal surfaces?

Thanks for help!

Ádám Komálovics

SolidworksSimulation