Thanks to a “AC Infinity MULTIFAN S1, Quiet 80mm USB Fan” ( available at the usual online mega sellers ), my 4K Roku Ultra runs cool( or not hot or even warm ) to the touch. This particular Roku model is a flat square that accommodates a fan on top of it. The fan plugs into the Roku’s USB port for power, which works just fine. It’s also at least whisper quiet. My last Ultra failed ( probably due to overheating but that’s only a guess - Roku replaced at no charge ) in 2018 and no issues since. HTH
In case anyone actually cares about former customers of Roku, think I figured out why my player has been bricked. Up to the latest version of HDCP (2.3 I think) it was always supposed to be useable by 1.4 HDCP equipment, just not in 4k. My TV is 1.4 HDCP compliant. Wasn’t an issue w/Rokus 9.x OS. just got bricked by 10.x. Apparently Roku decided to lock all their players to v. 2.x with zero communication with former customers.