These are stackable 3.5 inch hard drive trays. They have low sides and holes in the bottom to save on plastic and increase air flow around the drives.
They have screw holes to mount the drives. However, a lip on the front also keeps the drive from sliding out. So, I never used the screw holes.
A recess is provided to clear a standard SATA cable.
The two base mount strips can be attached to a surface with double stick tape and the trays then stacked on these, to provide secure storage. I mounted these right on the top of the computer enclosure.
Designed to be utilized in conjunction with https://www.thingiverse.com/thing:3177185 However, they can be used with USB to SATA adapters, external SATA cables, or simply to store drives not being used just as well.