Those bridge rectifiers do seem to have more losses than using individual diodes and need more heat sink. They are designed to run up to a junction temperature of about 150 deg C, that means that the base of the module can run up to somewhere like 60C allowing for the internal heat transfer.
For wind duty with high spikes of current it is better to keep the base temperature lower than this so as long as you can stand your hand in the base of the module comfortably don't worry if it is warm unless you expect to use it over the 20A you mention.
There are two things with heat sinks, the thermal conductivity must be high to get the heat away from the base and the surface area must be sufficient to dissipate the heat once you have got it into the fins. Commercial heat sinks are designed for convection cooling or forced air cooling, the proportions are different, a cpu sink is force cooled and will have the necessary conductivity bit will be low in surface area for natural cooling.
Cooper has a slightly better thermal conductivity and can be thinner for the main conduction away from the base of the module but you will need very similar surface area for natural cooling.
More important is that you get the best possible thermal contact between the metal pad on the module and your heat sink, keep things flat with no burrs and use heat sink compound. Unless you get the heat directly into the heat sink you will fry the bridge with only a low temperature rise on your sink.
I couldn't get at your picture but your description sounds adequate for 20A if the mounting is good.
Flux
