We have performed an unbiased search for the lowest-energy geometric structures of medium-sized silicon clusters Si-N (27 less than or equal to N less than or equal to 39) using a genetic algorithm and nonorthogonal-tight-binding method, followed by a refining and biased search using basin-hopping method coupled with density-functional theory. We show that the carbon fullerene cages are most likely generic cage motifs ("magic cages") to form low-lying stuffed-cage silicon clusters (beyond the size N > 27). An empirical rule that provides optimal "stuffing/ cage" combinations for constructing low-energy endohedral silicon fullerenes is suggested, with a hope that it can provide guidance to future synthesis of "bucky" silicon.