The better cases these days have rails in their drive bays. I always had a hard time implementing that in my own designs, but I think I found a solution that doesn't take advanced machining skills and equipment. I prototyped a unit for HDDs in plywood and it seems to work. Since I haven't seen the design anywhere yet, here it is. (Sorry, I don't have a fancy camera to take good pictures, so it is just the drawing.) Each drive gets two strips attached as shown at the top. Note that the strips stick out quite a bit at the "front" - that's the end opposite the connectors - and are a little recessed at the back. I used 0.25" thick material cut into 1.1" wide strips, so HDDs can be pushed together at 1.1" pitch with an air gap in between. Second from top are a front view and a sideways cross section of the cage. It is just a square tube with a backstop at one end (purpose explained later) and some tapped holes in the front for mounting the bezel. I dimensioned the cage to fit in 3x5.25" bays, but you can change that as you like. Third from top is the bezel that covers the cage. It is a little wider and taller, since that is what the standard for 5.25" bays suggests. The bezel also carries two fans. I have not implemented a plug (didn't have one), so it just dangles from the wires when I take it off. The 60 mm fans leave enough room for activity lights above and below. The figure at the bottom shows at the left side how the HDDs sit side by side to fill the cage. If you don't have 5 at hand you need a dummy block so the others don't fall around. The right side shows a cross section of the cage with HDDs and bezel installed. The rails of the HDDs are clamped between the backstop in the cage and the bezel. I was sloppy and the rails were not all exactly the same length. Some weather strips along the bezel and the backstops made up for that. It's not shown because I did the drawing first. The catch with this design is that you can only take out a HDD if they are sitting side by side. That may not work for all installations, but that's the only problem I encountered so far. It should be possible to adapt the design for installing 5.25" drives like CDs. Instead of the full bezel one would just have a strip of material at top and bottom to hold them in.
but usually these 60mm fans are extra noisy. isn´t it wiser to put one 120mm fan instead those 60mm ones?
That depends entirely on what fans you have. Usually you don't have to run the fans at full speed. For a test I used Everest from Lavalsys http://www.lavalys.com/products/overview.php?pid=1&lang=en to monitor temperature on some SMART drives. Running the two fans at minimum speed did the job. Warning: To lower speed I had initially connected the fans in series from +12V to -5V (17V:2=8.5V per fan) that made it very noisy because the electronic driver circuits for the motors were interfering. Series resistors on each fan did a better job. Since I started this thread I modded the bezel and used a 120mm fan to cool 4 HDs. On the very left I cut a slot and square hole for a floppy. It is a tight fit and I had to put a cutout in one sidewall for the fan, but its also a very compact module. Noise isn't any better or worse than with two 60mm fans. The 120mm fan has four LEDs that I wanted to use as activity LEDs for the four HDDs (two PATA, two SATA). I haven't figured out yet how to do that electrically.
