A boom accessory for a fork truck comprises a frame having receiver tubes for receiving forks of a fork truck, a boom extending forwardly from the frame, and a frame-to-carriage anchor assembly on the frame. The frame-to-carriage anchor assembly includes a carriage-attaching subassembly with top and bottom connectors for securely engaging mating structure on the fork truck's carriage, and includes a vertically-slidable anchor between the carriage-attaching subassembly and the frame to restrain the frame horizontally from sliding forwardly off the forks. By this arrangement, the forks can lift a weight of the frame, boom, and boom-carried load without stressing the anchor assembly, but can provide a substantial and secure horizontal retaining force prevent the boom accessory from sliding forward off the forks.