This construction discussion about HDD blocks has been done before. I believe there are two links
here and
here.
The second thread has examples of water blocks.
I've found that the major source of heat on a HDD is the MOTOR... not the sizes or the base itself... but the MOTOR part of the main casing. This is where a MAJOR part of the heat from the drive seems to be focused.
Some of the ICs on the controller board can get warm ... but I've never seen major heat from them.