Whilst I like the design and build of the BPN-350SAS there are some extreme flaws in the cooling design must likely due to implementation costs or interference with the units cosmetics.
Firstly, the 2x 60mm fans do not provide enough airflow; secondly, there are simply not enough paths for the air to flow. If you look inside the unit you will see 5 slits behind the PCB, and that is all there is for hot air exhaust. If you look at the front bezels, beneath the locking key hole plastic are 3 air vents, and this is all there is for the cool air intake.
Now, my solution has improved all off the above by first, removing the 60mm fans and installing 2x 80mm fans, these are connected via a molex connector. You can observe that 2x 80cm fans protrude 20mm on one side so it was necessary to install the protruding fan once the unit was installed into the case. Secondly, additional air holes on the back of the unit and on the top and under side of the unit to allow more air to circulate. I also removed the 5 springs that popped the drives out, but this was more to improve the backplane’s connection with the drives.