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Very little, but it would vary depending on the heatsink. Many are 3mm tall, plus the thermal pad, so maybe 6 mm totoal? Perhaps a simple square cutout just larger than the actual heat spreader to allow the heatsink to stick through would be the best way to go? This might require the mounting brackets to stand up off the surface a bit taller too, to accommodate for it sticking through. It would also allow a little air to flow across the back.