You appear to have 1.5" of total vertical space for the two drives.
The largest drives I can find that should fit are Seagate STEA3000400 drives, 3TB, and 0.58" high. (Ordered from Amazon, should arrive in a few days).
4 and 5 TB drives need 0.81" to 0.88" of vertical clearance. The additional capacity would be welcome.
If you have a larger drive, there is no reason the monument can't sit on top of it. In my view, the goofy "bay" is pointless, except to make the monument take up more space. I ordered the Seagate 8 TB drive, and it is working great. It would never fit into the drive bay.