Where I live, the mass transit agency puts bike lockers at train stations. These are rectangular boxes divided diagonally with a locking door at either end and are just large enough to accomodate a single bike in each half. They are rented out for 3 or 6 months at a time and they provide a key to the lock. They appear to be fiberglass or similar, with some sort of internal metal frame. The nice thing about this concept is that it's modular so it would be easy to expand if demand for them increases. They are bolted to the pavement from inside so there is no required construction and cost little to install.