My Galaxy S6 was overheating about every 10 minutes when playing games in the Samsung Gear VR headset. This uses a 40mm x 40mm x 10mm 5v fan (https://smile.amazon.com/gp/product/B074YJH3P9) with a usb connector, powered by a small phone battery pack. You will have to glue the fan box onto the back. With this I can play for well over an hour on most games without overheating the phone now.
Will have to print parts, then glue together, orienting the opening in the fan box upward to slip the 40mm fan into. The fan box stl has some extra spacers to glue in on the back of the phone adapter to allow more airflow if you want to use them.