I am using a very similar setup as you described. The heated bed is a 320*320 24V that uses ~19-20A on initial powerup and then slightly less as it gets warmer. Like you I already had a powerful 24V supply and would rather not use mains wiring for the heated bed.

This is the mosfet board I've been using to bypass the Duet:
https://www.amazon.com/HiLetgo-Printer-Accessories-Expansion-Replacement/dp/B07C7RQ4C6/ref=sr_1_7?keywords=3d+printer+mosfet&qid=1564695675&s=gateway&sr=8-7

It does not even get warm during heatup, and I've had no issues with it, I just drive the control signal from the Duet heated bed output. Let me know if you have any questions about the setup.

Edit: Just wanted to add that I use 12AWG wrire from PSU>Mosfet and 12AWG Silicon high temp wire from mosfet to heater.