Use as large a case as you can to prevent heat buildup inside the case.
Install the drives with an open bay above and below each drive.
And / or, install the hard drives in vented 5 1/4" drive bay adapters.
I have a couple from Antec that are designed to give you an extra HDD mount with USB ports, and have a vent panel on the front.
Install hard drive coolers on all hard drives.
Use the slowest drives you can find.
5400 RPM drives run a lot cooler than 7200 RPM drives.
Set the drive mode in BIOS settings to 1 or 2 steps slower than the drive supports.
Look at how the PCs are used. It may make sense to set the drives to spin down after 5 minutes.
Force a lot of room air into the case using filtered input fans.
I normally recommend that the inlet fans have 10 percent more air flow than the outlet fans. In you case, i would recommend 20 or 30 percent more air flow.
Depending on your environment, several companies make cabinet coolers.
These are basically small capacity air conditioners designed for cooling 19" rack systems.
One manufacture is Noren
Chas