If I were installing a cooling device like this in a loudspeaker with one of the cabinet walls very near the magnet, I might be tempted to make a large plate that acted as a heat sink, and connect it to a heat-conductive metal duct inserted deep inside the motor. This would allow air to pass through the duct, but would also heat sink to the plate. So for basshorns made this way, that might be a very attractive option. It's a similar system, but it uses the access panel as the heat sink.

Don't just place the heat sink near the speaker and hope convection will cool the motor. It won't. You'll need physical contact between the access plate and the vent, preferably via a metal duct that extends deep into the motor, and so has a lot of surface area contact with it. Also, I found that once a panel is placed within about 1/4" of the vent, it was shrouded so effectively that flow was reduced to almost nothing. So be careful that the vent isn't restricted and there is adequate flow.