Free-Free boundary condition means no boundary condition (i.e. the edges are free from tractions and moments). In static analysis it leads to the problems with rigid body motion and stiffness matrix singularity. In dynamics there are no problems due to inertia.
Our new article "Modal and Harmonic Response Analysis of New CFRP Laminate Reinforced Concrete Railway Sleepers" maybe usefull for you. In this article there is detailed information for the free-free condition modeling and validation.
One trick that may work is to start the analysis with a plate constrained by elastic supports on all edges. Specify the stiffness of the elastic supports, then gradually reduce the stiffness values on all the edges. As the stiffness values approach zero, your result will approach that of the free free boundary condition.