20A Fuse or thermal cutout on the motor supply? (Unless the supply safely limits current already)
1k resistors on all signal connections between Arduino and motor controller (extra layer of protection).
Keep high current wiring away from Arduino and signal cabling as much as possible.
Initial testing with a much lower current/voltage load (perhaps use 12V and power a 12V bulb), then work up to full voltage and then current.
Wear eye-protection if the controller board is not behind a screen, failed MOSFETs can explode.