Personally, I wouldn't worry. Drives don't get that hot. At least not until you start running a lot at the same time with no cooling or if you run a bunch of high speed (15k) drives. I have an Antec P180 case with 2 drives in the bottom compartment. There is no "fan blown" airflow over the drives, just whatever air is pulled through by the PSU (which has a fan on the bottom, not on the back) and the drives never get above about 42C.
\Dan