You don't need all these fancy resistors. I was stuck on this same problem a few months ago, looking up all sorts of 12v water heater elements... eventually made a perfect load resistor in 5mins out of steel wire wrapped around a plank of wood and then submerged in a bucket of water!
It was a good ~20amp load for testing my 24v batteries. I then made a second one and connected them in parallel to get more load. Worked well. I ran them for about 16hrs with a victron smart shunt to log the current over time.
I used cheap zinc plated tie wire, perhaps about 0.5mm thick, and about 5m of it wrapped around a wooden plank. I'll try to find a pic...
Ps. I realise the OP is looking for much more power but my point is long lengths of steel wire is resistive enough and the water keeps it cool enough that it won't burn out. I think you could make something similar to mine but with a much higher power.