FWIW, I used this:
AC Infinity MULTIFAN S4, Quiet 140mm USB Fan, UL-Certified for Receiver DVR Playstation Xbox Computer Cabinet Cooling
www.amazon.com
It's an AC Infinity 140mm fan, it's quiet, had a speed switch, rubber isolating feet on both sides, so I just have my '18 Mini sitting on it (and it's very stable), with the bottom cover still on (I may remove it one day and do a few tests). Definitely helps according to Macs Fan Control, and it's a very high quality fan with a lot of air movement.