After my previous attempts I realized that although my cover designs where really silent using Noctua 40mm fans, the print results were not as good as it should be. Noctua 40mm had not enough air flow to cool PLA parts as they were printed resulting in poor quality bridges and overhangs.
So I've tried an opposite approach. I've replaced the quiet part cooling fan for a heavy duty 7530 blower with a cooling duct tightly near to the hotend output.
On my first printing attempts it seems that quality has really improved on parts that with my other approaches failed or resulted unsatisfactory.
I've kept the fastening slot for a BLTouch from my last design.