Some further thoughts:
The dissipation will depend on which way up you use the box and what it
is standing on. If it is hemmed-in by other equipment or might be
accidentally covered with a blanket, you need to alow for much-reduced
dissipation. If there is any sort of risk like that, incorporate a
thermal swich.
The colour of the box makes a difference, but not in the way generally
supposed. Painting it black will make almost no difference to the heat
dissipation because it is not going to be very much hotter than its
surroundings, so the net radiant heat loss will be trivial. However, if
you are going to use it outdoors in sunshine, painting it black will
make it hotter as it will gain heat from the sun (which is at a much
higher temperature).
Leaving it with a natural 'silvery' finish will be a slight improvement,
but this may not be very reflective to infra-red, so it will still heat
up (it gets worse as it becomes old and grey). Some sort of sunshade is
the simplest answer.
Jeez guys. This is simple. You make an Aluminum block INSIDE the box
that is BIGGER than the tab on the transistor (like double or triple
wide). You use thermal epoxy to attach that "heat spreader" to the box
permanently.
You attach the heat source (transistor) to it with ideal means as well.
Then, you have to put a heat SINK on the outside of the box to grab the
heat that the heat spreader has passed through the box's thin wall now
over a greater area due to the heat spreader inside the box. This is the
key to success here. The heat spreader. Thin wall boxes suck at
delivering heat conduction flows well. If it is custom, you can have the
spreader cast in, removing yet one more lossy interface from the system.
The external heat sink can be a finned device meant to remove heat by
way of air currents (convection). OR it can be a conduction cooled
paradigm, where you only need to insure that a good thermal conductor is
against the side of the box that has the heat source. Of course, the
whole box will end up with a higher runtime temp, but the key here is
conduction of the main heat source to the outside of the box.
The footprint of the transistor tab alone is not enough because there
are multiple interfaces to lose conduction through, and the tab just
isn't enough. The heat spreader makes GOOD conductive (thermal)coupling
with the transistor tab, and SPREADS that heat out, so that the
attachment to the can is greater, and therefore the efficiency of any
external heat sink is greater.
No math needed here. It is simple mechanics. Big solder tips heat
faster than small solder tips. It is about thermal mass, and how you
present that to where you want the heat to go.
If you want to remove heat from a source inside the thin walled can,
you need to spread that heat out INSIDE the can so that you can more
effectively couple it to the outside of the can, so that you can then
carry the heat away by convective sink attachment or conductive cooling
sink attachment mass which you mount it against..