A platform design including a module black-box instance is loaded into computer hardware. Using the computer hardware, synchronous boundary crossings between a static region and the module black-box instance of the platform design are identified and objects of the platform design included in the synchronous boundary crossings are marked. Using the computer hardware, unmarked objects are removed from the platform design to generate a shell circuit design. A custom circuit design is implemented based on the shell circuit design and timing constraints corresponding to objects remaining in the shell circuit design.