As clearly stated by Icy Dock 'ease of install' is a primary feature of the MB982SPR-2S with its screw-less design. The first stage is to open it and pull back that upper drive tray.
Ready for drives.
Once clear you can pop the first drive in place. It just sits in the space since the act of closing that main cover actually shifts the drive assembly forward 'docking' it with the SATA sockets within the enclosure.
The upper drive tray is then lowered over the first drive and the second drive lowered into the available space.
Then you simply close the main cover - you may be able to see that the position of the transparent tongue has shifted towards the rear of the enclosure and is now over the enclosures SATA PCB. In the previous picture it wasn't over it at all. That illustrated how the drives are pushed towards the rear and plug into the SATA sockets.
Plugs in as it shuts.
When closed you can see there's a bit of airspace between the HDD and the enclosure casing.
Airspace for vetilation.
To check their claims of compatibility I used a 3x3.5" Icy Dock drive enclosure in my own PC. I first installed the guide rail assembly onto the drive with no issues. Those mount holes do mirror standard 3.5" drives.
Icy Dock guide rails.
Then slid it successfully into place. No issues and when powered up the drive was recognised correctly and functioned perfectly.