So, in installed the proper FAN (Noctua).

In the attached pictures you can see

1. Cooler Master temporary FAN, case open. With this setup I achieved 29/31 °C temperature, almost constant, for the WD and Seagate drives respectively.

2. Noctua NF-A9 FLX Fan, which I bought as a permanent install.

3. Nocuta included Low Noise and Ultra Low noise adapters for the FAN. I installed the Ultra Low Noise adapter, which is of course also the one producing less cooling.

4. Noctua in place. Notice how, being smaller, I could push it further in. I am hoping the fan can more effectively pull air from the bottom holes you can see in the picture when the case is closed.

5. Case closed. As you can see, there's NO ventilation holes whatsoever in the side panel.

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After 30 mins up and running, case closed and Noctua FAN spinning at very low speed (Ultra Low Noise adapter), HDD are operating at

WD 34 °C
Seagate 35 °C

If they stay like this, I am going to bee happy. We'll see in few days.