This is a very minimalist design for a HDD caddy I came up with when I needed to cram four 3.5" HDD's into a tiny case for my home NAS. It works well for a four disk array and I included a single spacer just in case anyone wanted to stack higher than four. I have successfully done up to six however I do recommend 35% infill when going that high. After all HDD's can get heavy.

The edges on the HDD support spacers are at 5mm which permits plenty of airflow between the drives under moderate working conditions. (My case had a 120mm fan directly behind pulling air out which worked perfect) The 5mm support spacers are also chamfered on the bottom edge as I found out some 3.5 drives have impressions on the top that may interfere with the spacers.

NOTE: Drives can't be installed upside down based on screw hole and spacer support position.

This is for a flat mount HDD setup like the one you see in the picture.