Reason behind geometry of heater blocks? I understand the principle of why a heater block is used. Helping to reduce temperature variation as the filament is extruded using the heat capacity of the block.

But I’m wondering why it takes the form it does? I imagine it is cuboid in shape just for convenience as it’s easy to machine?

From a surface area to volume ratio a cuboid appears to be one of the worst shapes to use.

Note:
I am currently doing a project which requires me to increase the tool clearance of the nozzle of a 3D printer. Hence why I am exploring alternative print head configurations and heater block design trying to minimise the profile of the print head as much as possible.
A heater block has its shape designed simply by the necessity of the needs and the ease to manufacture them. Functionally, a heater block serves as:


a structural component holding the following components in a well-known position to one another:


heatbreak
nozzle
heater cartridge
thermosensor

a transmission medium of heat energy from the heater cartridge to the thermosensor and filament path.


this dictates the use of high thermal conductive metals  

a thermal energy storage to equalize the heating pattern of the heater cartridge.


While this dictates the internal geometry of the block, it does not say anything about the external geometry. This is chosen entirely because of manufacturing, which strongly prefers square items in a vise with stop positions to allow repeatable machining.

While other heater designs are existant, for example wrapping a heater wire around the meltzone, the use of a machined structural element that houses bought components saves a lot of time in assembly Quality Control and subsequent repairs, as each component (heater, thermosensor, block) can be rejected or replaced on its own. This makes the machine all in all more serviceable.