Coretechs' 4 X6500s (rev 2)
"There is good airflow over the top and bottom of all the boards as it sits. I used 1 1/8 in. standoffs ($6 for a pack of 25 on Amazon, might be cheaper elsewhere) that seem to be almost the perfect height for using 2x 80mm fans. +1 piece of card stock."
The last photo shows coretechs' first setup, later improved to the more compact 4 board stack.