[Patrick]

I don't think you could do it, not without getting one custom made to fit the shape of the fins (obviously it would only cover one side of the motor, because if you made one to go around both sides it wouldn't slide on).
The easiest way would probably be to use the motor mount as the heastink, even though it wouldn't have as much of an effect.
Usually if you have heat issues with a neu either somethings wrong with your setup, the motors not suited to the setup or your pushing it really hard, because neus don't get hot that easily.

Another guy at racing runs a finned hacker in his kyosho truggy, and before he got a rcm motor mount he used to have the motor wrapped in alfoil and clamped to the chassis with piece of thin aluminum bent over the top of the motor. He used the aluminum to fill the gap between the fins, not sure how well it worked though, the motor still got hotter than my neu.