I took mine apart the top plastic pqrt needs to stay off and attach heatsinks to the metal Roku heatsink attach heatsinks with thermal materal. I can see why everyone is having problems. The bottom plastic piece has only a cheap thin metal strip with a blob of non conductive paste making contact with the pcb.
Mine only reaches 95c tops steaming several hours of 4k video now opposed to 115c prior to the mod.
That would help if the thinking was the problem is due to approximation to other devices radiating heat but sadly that's not the case despite what Roku is peddling the units cannot handle the heat they are generating . ie a design flaw !
Have had this streaming stick + for less than a year. Tonight watching a 4K show on Prime Video and got the heat warning message. The stick is plugged into an HDMI port without an extender and gets power from a USB port. Changed to the non-4K version of the show and had no further problem.
Ordered the extender from Roku and hope that resolves the issue.