Well, you can *buy* a 4-in-3 module. I have two stackers, and one has
6 hard drives (5 raid + 1 os), and the other has 6 hard drives (1 linux, 1 windows, 4 data). There are not many cases that can do a decent job of cooling 6-8 hard drives. If you know of a cheaper one, I would really like to know about it. There is the antec p-180, but it maxes out at 6 hard drives, and it has those clever sleds for the top 2 hard drives that cover the bottom of the hard drive (the hottest part), which will limit cooling. Perhaps I am overly cautions, but I like to keep my hard drives cool. And for those that ask, yes I have 10k and 15k rpm scsi drives.
I think the coolermaster stacker is a bit complex, heavy, big and expensive, and could be quieter, but it is unsurpassed if you have more than 4 hard drives you want to keep cool.