The Parallella cluster stack packs 144 processors and 200 GFlops into a space of approximately 150x86x54mm. While the Epiphany processors are very low power, the Zynq FPGAs hosting the ARM cores and glue logic get warm and need cooling.
I've published a design on which provides a holder for two 120mm fans to force air across the FPGA heat sinks of the entire stack. The design is configurable to allow for different size fans. We produced ours using a RepRap Prusa-Mendel 3D printer. It is effective at keeping the FGPA to around 50C, even in a warm office.
Enjoy.