Spawn a mob on player-built structure

Hi, I need help with spawning a mob when a player builds a specific structure (kinda sike spawning an Iron Golem). I've seen multiple tutorials on how to do it and they all check for the structure block by block after the trigger is placed, but my structure is quite complex and that would be tedious work and the bugs are probably inevitable. Is there a better way of doing it? Thank you in advance!