Glacier-style cold storage

Design a Glacier-style archival cold-storage system optimized for cheapest-possible long-term retention of rarely-read data: think compliance archives and backups, write-once and read-almost-never, eleven-nines durability, and retrieval latency of minutes-to-hours being acceptable. The trick is that storage cost must be a fraction of hot object storage. How do you achieve that cost while keeping data durable and (eventually) retrievable?