Determine whether chassis is a suitable heatsink?
Using the casing as a heat-sink is a common method however a few things must be understood.
As Laptop2d mentions it is hard to model the thermal characteristics of the case, and an experimental measurement may be prudent.
Heat-sinks rely on air-flow to work. Since those plates are flat, there is a good chance that someone will install the box butted up against something thermally insulated... for example, pushed back against drywall. If this is for something of your own, and you can control the airflow, it may be fine. Otherwise you may need to add features to the plate to prevent the occurrence and design it work under those worst case circumstances, or things may fail or even catch fire.
How hot will the plate get. Though the heat-sink may be sufficient to keep your electronics working, the plate itself may be very hot to the touch, even hot enough to cause skin burns. It is important that any exterior surface be kept at reasonable temperatures.
Physics dictates that the plate will expand under temperature. This can result in unfortunate mechanical side-effects in some cases. (pardon the pun...)
Your side panels are classical examples of the simplest form of radiator - the flat plate. Simple equation for calculating its thermal resistance (in W/K) is: $$R_{th} = \frac{3.3}{\sqrt{\lambda \cdot d}}C+\frac{650}{S}C$$ where:
\$d\$ - plate thickness in mm,
\$\lambda\$ - thermal conductivity (237 W/mK for aluminium),
\$S\$ - area of plate, in cm\$^2\$ - in Your case You should assume that only one side of plate dissipates heat,
\$C\$ - coefficient which depends on plate surface and positioning: 1.0 for horizontal raw plate, 0.85 for vertical raw, 0.50 for horizontal blackened, 0.43 for vertical blackened.
Of course, as always as we're talking about heat transfer there is no one simple answer, because most of equations in this field are empirical. For (probably) more accurate solution look for example on this article: http://www.heatsinkcalculator.com/blog/how-to-design-a-flat-plate-heat-sink/
The problem is you will need to model the whole box and the air to come up with a reasonable figure for how much heat the chassis can bleed off.
You could just model it like an infinite thermal sink (at room temperature) and then use the thermal junction coefficient of the package and the thermal resistance of the thermal paste or pad you are going to sink to the box.
Or if the plan calls for dissipating a lot of heat the box could be modeled as a thermal resistance. Aluminum is 205.0 W/(m K) but the problem is the air is surrounding the whole box so to really model this you'd need to sum up all of the thermal resistance at many different points because the air has a thermal conductivity of 0.024 W/(m K)
From experience it would probably be just be easier to attach a resistor to the side and measure it.