I replace the case fans of biqu b1 with this mod.
Parts needed:
1x buck converter (https://www.banggood.com/LM2596-DC-DC-Verstellbar-Step-Down-Schaltregler-Power-Supply-Module-p-88252.html?rmmds=search&cur_warehouse=CN)
1x silent 120mm fan (http://www.dataspot.ro/produse/Ventilator-Scythe-Kaze-Flex-120-PWM-SU1225FD12L-RDP.php?specids=4763,4764,4765,4768,4769,4774,4776,4777&producator=254&spec_4763=120%20x%20120)

You will need additional tools to cut the aluminum cover case. The cut needs to be 123x123mm.

When I have more time, I will add to this also the drawing of the cut in the case.

Important: For this mod to work you will need to replace also the PSU with a slimmer model. The one that I use was LRS-350-24(https://www.meanwell.com/productPdf.aspx?i=459)