"The cards are rated for ambient temperatures of up to 55 °C and chip temperatures of up to 105 °C. Without proper cooling the case will get too hot to touch (and then fail)! Usually a small breeze coming from a fan is enough to keep the temperatures in check. However, I was looking for a fanless solution, so I came up with this: Two big-ass heat sinks (4.72” x 2.72” x 1.06”), which I got for $17, stuck on with 6.0W/mK 1mm thermal pad, cut to size ($5). The heatsinks are just a little longer than the case, allowing for some air exchange when put upright."
Source: Dr. Christian Kohlschütter
