Here is a picture of the solution I came up with. A friend of mine made it on his lathe.
It is an aluminum part with a thickness of 12,05 mm. Smallest dia = 20mm, largest 45mm. We put thermal paste on the processor to position it as accurate as possible. Then with some scrap thermal adhesive, we -as accurate as possible- put the top cover on. Then we took the lid off, which now held the aluminum part. Drew a circle around it and then stuck the part with the thermal adhesive on the exact place in the top cover. The result is amazing. So far I have not seen it above 45 degr. wiht the total processor activity around 50% during rebooting/indexing and playing a move. Music playing is at 35-37 degr.
We considered the following:
For best thermal conduct, completely cover the surface of the processor.
For best contact, have a large surface to connect to the top, leaving space for air for the surrounding electronics.
For good level contact the allu. part had to press the chip slightly down as it was not exactly horizontal due to the spring that holds the board.
The spring pressure of the board in its holder would take care of the necessary room to press down on it and level it out.
The exact height of the part was determined by putting it on the processor and closing the slice to see if without pressure the part would stay in place. The tiniest space between body and cover was allowed. After we had decided that 12,05 was correct we assembled it with thermal tape of 0,5 mm thickness. With some fresh thermal paste applied on the processor. When we closed it there was just a minimum of play between the different parts of the slice body before we tightened the screws.This ensured that the allu.part was actually pressing the board a bit down.