HDD / SSD SATA III Adapter for MacBook (Pro) SuperDrive + USB box replaces Slim (money) for SuperDrive - 9.5 mm trolley kit (SATA - SATA) - TheNatural2020 (Personal Computers)
I was first out on much more expensive solutions, people who have set up shop, and then I found this kit, I tried it and I am far from regretting my choice.