Step up from low-end thermal pads supplied with your GPU waterblock (typically in 3 mk/W range) at reasonable cost. Extra soft composition works well for blocks with imperfect tolerances. This model is available for thicknesses up to 3.0 mmColor:
White-GreenSize: 100x24mmThickness – 1.5 mmPropertiesUnitToleranceTest MethodThermal Conductivity 12.5W /m• K± 10%ASTM D5470 Modiï ¬  edFlame RatingV-0– – UL 94Dielectric Breakdown Voltage>10KV/mm– ASTM D149Operating Temperature-50~+180° C– – Hardness55Shore OO± 10ASTM D2240