Bear in mind it's the sum of the thermal resistances of the device itself, its case, any mounting washer and the h/sink that have to be added to determine the power rating. Just fitting a larger h/sink to an underrated device soon gets into a law of diminishing returns.
For this kind of duty, avoid TO220 devices as you just can't get an adequate thermal bonding to the h/s on so small a case. TO247 or TO3 will be a better bet, though even so one would be stretching things, think in terms of at least two in parallel.
Going fan assisted will definitely lead to a smaller and probably cheaper unit than a monster convection h/s. Thermostatic control them becomes preferable to avoid too much noise on light loads. Although that does call for a separate fan supply, of course, which convection cooling avoids.