There are several possibilities to improve the cooling in addition to enlarge the heatsink:
-use fan mounted on the heatsink, or even fixed in the cabinet side: since the temperature gap between measured and max authorized is small, this will be enough,
-any other semiconductor devices around ? Take them away... A good design of cabinet = a good distribution of each heating devices to avoid "hot spot",
-check the heatsink fins orientation: they must be vertical, not horizontal,
-check if the test conditions are similar to the field conditions (field conditions is usually not 100 % ON state).