Thanks @g7ruh and @shutterbug. My experience has been along similar lines - while I was scraping the library afresh, the CPU had to throttle down (Kodi even displayed the dreaded thermometer icon!).
I'm not designing any special heatsink. Another Slice user - @rahuljawale - has worked out a 15mm x 15mm x 12mm copper block that I intend to get and install. What I was looking for more was along the lines what would be an appropriate temperature to aim for - to not risk shortening the life of the both the CM and the hard disk. I experience ambient temperatures of around 28-30 deg C, and CM1 used to top out at 67 deg C while playing 20GB MKV files.
If I end up with 67 deg C, I'd try to work out placement to cool the box after heatsink installation to try to take it down further.