I found Wired1's solution https://www.thingiverse.com/thing:29572 most suitable for my use. Then the heatsink I ordered arrived and I wanted to add that in for an additional cooling effect.

(FYI, the addition of a heatsink to the motor only reduced the motor temperature by around 5℃ to 8℃ for me, whereas adding a fan lowered the motor temperature by around 20℃.)

Minor changes:

1. Increased the depth to allow <12mm heatsink to be pasted on.
2. Added some holes in the hope of increasing airflow. Inconclusive so far.
3. Changed the mounting hole to take M3x3 (M3 OD 4.5mm, 3mm depth) threaded inserts.

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