ON THE PLANKS of the attic floor waited Frederik’s secret: a town hunched on an island, a heap of netting that had washed into his path once when north winds drove the waves clear across the causeway.

The houses were made of empty boxes that he’d lifted from merchants’ rubbish bins. Frederik cut out windows and folded the cardboard: perfect hinged shutters. He built eaves out of slates that the wind had liberated from real roofs. He planted trees by poking sprigs of balsam into dollops of boat caulking. Best was the customhouse: a gold-papered chocolate gift box sporting a porcelain dome—an upturned bowl of chipped blue china.