I made pictures of the new temporary setup with real heat sinks. The top one is pushed between the powerplug and the SFP and the lower one rests on a dummy RF45 and to press the heat sink up there is a washer between them. Because there is not a perfect fit for the lower one I have applied some thermal-paste. In the final version I will glue the heat sinks to the SFP.
The measurements of the heat-sinks are 2,5cmx2,5cm and the thickness is 5mm. To give the power plug more room you can file in the middle some material away.
Update: I have installed a small 5cm fan to blow over the top heat sink and the top of the hEX S and the temp of the SFP is now stable at 39 degrees and that is also the temp from the hEX S and which is 3 degrees lower than before. The moving air is also drawing warm air through the top holes of the hEX S
Update 2: the fan is so efficient that the temperature of the FSP is below the temperature in the box .