I just built my 3rd (dual)HD block. My first was a drilled top/bottom block. The second was a rotor inspired side side block. I built the 3rd with 1 thing in mind, minimal resistance. Its a slight improvement on the pipe soldered to the drive cage idea. I ripped a sopper pipe, and 2 90bends in half. Then soldered them onto a 1/8 plate.
It barely effects flowI have my HDs perpendicular to the case, hence why the barbs face out. The downward taper is to simplify/shorten the tubing of the system. They keep the drives at about 7C over ambient, about equal to my previous designs.
My next attempt will most likely try to incorporate the HD cooler into part of the loop. I'm trying to figure a way to use a HD cooler as a res or just streamline it into the plumbing the rest of the system.