There is ZERO air flow to the hard drives. The drives basically mount behind the motherboard where a wire gap would normally be. They didn't even bother to vent the top of the case which would have provided some ventilation. It is basically a hard drive coffin.

I love the case and have considered a few options.
Unless you are doing a water tank cooling system, there is plenty of room to construct a hard drive cage (or possibly buy one already built). I am looking for one now which is how I found your post. The second option would be to mount a couple of hard drives below the case. The stand gives it plenty of space though you would have to construct a bracket to mount them. This is easily done with a strip of aluminum you can find at just about any hardware store. How good it looks is up to you.

Because the back is designed to hold a radiator or 120mm fans, I am considering this case rack and just removing the power connector. I am looking for one designed to go in the case.

https://www.amazon.com/Sans-Digital-HDDRACK5-5-Bay-Organizing/dp/B001LF40KE?th=1

Anecdotal evidence / limited testing, server has been up for ~20 minutes doing a scrub and the drives are at ~31-32C in a room with no AC and outside temp of 77'F at night. YMMV

https://www.amazon.com/gp/product/B001LF40KE/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

^ The external rack