Thinking about this further, as an exercise, I wrote a preliminary schematic for a modular, high current, high power load based around the IRFP250. I'm not sure if I can push more than 100W per module but you can just add more modules until your current, voltage, and/or power needs are met.
Just stack (parallel-up) multiple modules across same POSITIVE and NEGATIVE banana jacks, I SET (P2.pin1), and P2.Vcc, P2.Vee, P2.GND pins. P2.pin2 and P2.pin3 are for future Arduino/controller monitoring of actual current and heat sink temperature.
However, be careful with cranking the pot to max. Make sure you don't exceed 100W per module limit. For example, if you parallel 2 modules, you will be able to pull 40A combined @ a max of 5V (for a total of 200W). If you have a 10V input, then the pot better not be cranked beyond half way (each module: 10V * 10A = 100W).
Good luck with yours.
EDIT: Made some corrections to the attached schematic for future readers.
IN-LINE ATTACHMENT REMOVED. SEE EDIT2 BELOW.
EDIT2: Due to some screwup with the forum software, when clicked, the above attachment is replaced with an image belonging to a later post of mine. I'm leaving the above attachment as is for the admins, if they wish, to investigate. In the mean-time, I've re-attached the correct image at the end of this post.