I have created a hard drive cage for the Lenovo P520 Workstation that slots into the secondary drive cage position. The cage supports 2 - 3.5" drives and 1 - 2.5" drive. The bottom slot will only fit a 2.5" drive if you install the drive before you mount the cage in place. The cutouts along the side provide an area to screw mount 3.5" drives. I found it easiest to slide the drives in and slightly bend the mounting cutout towards the drives. Cage mounting holes can be used to directly screw the cage to the side of the case or are perfect for aligning brass inserts. Added the design file for Fusion360. Please share and remix!