While separating the garbage I found an old tupperware plastic box. I smiled and remembered my first desktop which had an air intake with sort of the same shape. So I dove into my PC spare parts box and found a 120mm fan and ... the plastic box fitted perfectly around it. So I cut out the bottom of the tupperware box and placed the 120mm fan with plastic box on the case fan mounting right above my CPU.
The extra case fan blows air (from the room) to the cpu fan and the plastic box inlet prevents this (cooler) air to be mixed with the (hotter) air inside the PC. Room temperature outside the PC is 20 degrees Celsius, HWinfo says auxiliary temperature inside the PC case is 32 degrees Celsius (measured on motherboard). Because the CPU fans sucks air from outside the PC, the initial temperature of the air flowing over the fans is 12 degrees cooler, this explains why CPU is 7 degrees cooler (was higher, but I changed fan profile to 'silent' in the BIOS). CPU fan now runs at 1100-1200 RPM (in stead of 1700 to 1900) and the extra 120mm case fan runs at 650-750 rpm.
This el cheapo CPU cooling trick still works
(more important with less noise
)