Prius C owners - I need some help please..

I own a Prius C one and I am installing a center Storage compartment/ Arm rest between the front seats (The C one doesn't include a center storage compartment).

I need to know if the black plastic base of the Storage Compartment/ Arm rest unit is secured firmly to the floor of the Prius C, or is there a loose wiggle around the base (in fact can you slip your fingers up under the black plastic base "skirt?") -- or is it really tight and solid?

Mine is loose and not firmly attached to the floor by the rear passenger's feet -- (To further clarify, there's a cup holder that unfolds out from the back of the central storage compartment and it's down around the base of the unit near the floor).

I would really appreciate your help - I think I am missing some parts.