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...)