After fiddling with the control cavity master template for a while, I finally got it right and copied it into the final control cavity template. The outside template is oversized so I can use a copy ring – the shallow depth of the cavity cover does not allow the use of a ball bearing router bit (I only have them in an inch length).

I’m pretty happy with the result:

So, onto the control cavity cover. I thought I’d repeat the same process as with the neck pocket template, as you can see here:

On the left the DIY copy ring of the size I needed and on the right the control cavity size with a channel routed on the inside, producing an 1/2" undersized control cavity cover shape.

I attached another piece of MDF to this undersized cover and cut it to size using the copy ring setup. Due to the sharp corners on both ends I needed to touch up those spots with some extra wood and some filler. After that, I got this:

I’m pretty much satisfied with the result, although I need to sand it down a bit before I transfer it to the final plywood template… if I wouldn’t do so, I would never be able to remove the cover once it’s in the bass (without damaging it, that is).