just having a fan infront/behind the HDD cage works just as well as having the coolers under the HDDs tbh, I have three drives all in the same cage enxt to eachother and the middle one sits at 36ºc and hits 38ºc very rarely, I just have a fan behind them at the back of the case (exhaust) which seems to do the trick without taking up extra room using those HDD coolers.
Just get a good case with good cooling or splash out a few quid for some HDD coolers and you will have no problems
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