there is a few ways to do it depending on what level of skill you have you need about 2 ohms to get approx 20a you will need to dissipate neary 1kw of heat coiled up magnet wire works well if you keep it under water but i would only advise that if you know what you are doing ( i use that for testing 100a on 48v packs you would be suprised how fast 4.8kw will boil water )
if you decide to go the magnet wire root use this https://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-table
