I stumbled on to Kerry Wong web blog when I searched for some ideas on an electronic load.  Kerry also references a tutorial on an Electronic Load for testing batteries by Dave of EEBlog.
Kerry uses a lm324 to drive 6x fets. As I am wanting a lower capability, I will be only be using 3x fets.

Schematic

 

I am aiming at the following Specifications:

The heat sink is 100x50x15mm. I choose this one as it is easy to retrofit 2x 40mm fans.