ToughArmor EX MB491SKL-B:
Inspecting the MB491SKL-B shows that it is very similar to the two bay version. The entire housing is metal and looks/feels well made. It is also passively cooled, but has plenty of openings for air flow through. The door is secured with a thumbscrew and a keyed lock, but there is one different feature on this unit that was not found on the two by unit... While not shown in the photos, when you remove the drive tray from this device, and black plastic door springs down to seal things up. So, without a drive installed you do not have a drive sized opening in your case, which will keep out dust, fingers, etc.
To access the drive cage, simply unlock the lock, and loosen the thumbscrew. Then hinge the door open and pull the whole assembly out. With this unit, you will find the tray has a full metal frame that goes around the drive. This makes sure the whole assembly is the right size to slide in and out of the MB491SKL-B properly.
A typical 2.5" drive should be able to fit in to the tray without any effort, and then you simply screw it to the bottom, as shown in the image below.