A single logic-level N-Channel MOSFET should work, assuming you don't need to run in reverse. Connect "Source" pin to 24V Minus and Arduino Ground. Connect "Drain" pin to the motor's Minus side. Connect the "Gate" pin through a 1k resistor to an Arduino PWM pin. Connect the Plus side of the motor to the 24V Plus.
ON Semiconductor 2SK4043LS has a logic-level gate and can handle 20A and 30V.