The Control Cavity

Using the holes drilled from the front, I was able to align the template for the control cavity.  Here is the cavity template raised above the back just enough to route a cavity deep enough for the plastic cover plate to sit flush with the back.

I routed this with a pattern-following straight bit.
Here is the control cavity with the "plate recess" routed in to the back.  Note the holes on the top plywood template -- they aligned with the holes in the body to align the routing template.

Also note -- this is the same template I used to shape the control plate out of black plastic from Stewmac.com.
Next, I could "hog out" the remaining wood in the cavity using a Forstner bit and my drillpress.

Be careful not to go too deep, and take your time.