Here is a link for the adapter for your project. You should use a standard 9.5 laptop drive.
Add a hard drive to your laptop by replacing the optical drive with a second hard drive. Compatible with all Unibody MacBooks and Unibody MacBook Pros (excluding MacBook Pro Retina). 9.5 mm Internal SATA Optical Bay for SATA Hard Drives. — 50+ available at 39.95 each.