Soft compliant robots and mechanisms have generated great interest due to their adaptability, and inherently safe operation. However, a systematic synthesis methodology for these devices has always remained elusive owing to complexities in geometry, and nonlinearities in deformation and material properties. This paper builds the groundwork towards a constraint based design (CBD) method for a unique class of soft robotic building blocks known as fluid-filled fiber-reinforced elastomer enclosures (FREEs). First, the constraint behavior of FREEs with varying fiber angles is mapped using an automated mobility analysis framework that is based on matrix-based kinetostatic methods. Specifically, such an analysis seeks to establish the constraint behavior of FREEs as a function of not just the global geometry, but also its local anisotropic material constituents. Then, the paper demonstrates the principle of reconfigurable constraint by combining several FREEs in series in accordance to the rules of constraint-based design. Eventual extension to actuating FREEs will enable a comprehensive synthesis methodology for soft robots.