Although the housing router Mark mentioned worked pretty well, and was safe and accurate, it was also heavy and slow. We've found that a custom template for each housing shape is pretty easy to make, and actually faster to cut with. Here are 2 videos that show one of the templates being used in our shop:

The first one is the actual routing . Note that he's taking several passes, going a little deeper each time. There's a depth stop set for the final pass. Note also that he's only routing around the perimeter of the housing.

The second video shows cleaning up the housing with chisels. Note that each step took less than a minute.

Take a close look at how simple the template is. The base of the router just runs up against the wall of the template. There's a fence that hangs over the edge of the timber, which positions the template. The fence is longer than the template is wide, so it can be used to clamp the template to the timber. We're just using a 1" straight bit - no bearing - nothing fancy.

PS: I made the videos with my cell phone. You can play them with quicktime.