Copper conducts heat faster than aluminum, so you can use copper bar stock to mount multiple fans to each heat source. Aluminum transfers heat to air slightly faster than copper, so you still want copper at thermal source and aluminum heatsinks at the point of airflow. You can use thin enamel layer or another non-conductive paint to prevent ionic transition between the two metals. JB Weld works, too, but neither it nor paint will permanently secure them, so don't use them as glue. I used JB Weld on a Pentium 1 cpu and about 18 months later it separated like a rocket, bouncing off the ceiling, since the case ran without the lid on it. Funny enough it thermal throttled and continued on for more years with a better glob of JB Weld.
I also suggest never to mix airflow, else one hotspot feeds another. Thermal transfer through metals is much too slow to run inline like that. Give your heatsink intake through the c-channel via holes and not from an end like that. Spread those relays out and give them their own exhausts.
