How to make a dummy load of 1000W - 2000W?
It's probably cheaper to buy several 120V AC kettles. A single kettle will be about 2 kW when powered from 120V AC but at 48V DC this will be about 16% of the power at 320 watts.

Three kettles full of water gives you 960 watts. Add more kettles if you need more power.

Bonus, after doing the test the water may be hot enough to recycle or even make a cup of tea from.



Addendum

It's worth mentioning that the kettle heating element might be 10% lower in resistance when cold so there may be a a peak in power of 10% falling to rated value over the period of 1 minute or so.