I use porter cable's template set.
http://www.build.com/porter-cable-42045-...;baid=501887902 It is a steel or brass fitting that mounts to the base of the router and allows you to use a straight bit for all of the cutting. The fitting acts like a sleeve over your bit, and allows you to take multiple passes at different depths for your housing. It has an offset so you jig will have to be slightly larger than you housing. We just make a simple jig of 2x2's on either side of the timber and half inch MDF to make up the jig. Just 4 pieces of MDF screwed down with a couple of drywall screws, this setup is then clamped to the timber and the routing begins. The porter cable template rides along the MDF and bit cuts roughly an 1/8th inch from the MDF. We have a few standard jigs (standard brace, 8x8, 6x8, etc.) but we often make new ones. There is a stack of MDF pieces in the back of the shop just for the jigs.
