Awesome work! Probably took as long to produce the video as it did to build it.
This is a version 1.0 design and the nitpicking (while valid comments) should take into account that this is a first effort. I think it's an incredible feat. There are only 2 things I would change for v2.0 that would make this even better:
1) Accessibility: I would mount each drive into a removable tray. Changing the drives out is going to be a bear. Having removable trays would not only make this a snap but also allow for hot swapping, a real help when dealing with RAID failures. If you can get a tray with built-in heatsinks for the drive that will lead nicely into my next recommendation.
2) Heat: The heat issue should easily be solved by putting heat-sinks on the drives (or heat-sink trays as mentioned above). Then simply seal the box, put vents on the front left side and channel the air through the front left, across the drives to the right and through the back. The drives will need more spacing to accommodate the trays and airflow but maybe you could switch to 2.5 drives. This will definitely take some engineering, especially to work around removable drive trays, but proper sealing and a good fan in the back would give a good airflow. May need to increase fan speed or add additional fans to the front intake.
You're definitely up to the task.
Awesome job!