I wanted to add audio to my MGC in a manner that would limit the outward appearance that there is an audio system installed.  Bluetooth provided an easy solution to the problem but where to mount the speakers became my top concern.  I decided to build a box that would sit within the dash speaker location to mount a set of 3.5″ speakers.  This video shows some of the processes I used to make the speaker box and how I mounted it within the car.

Here is the list of parts I used, not including the wood:
Toggle Switch: http://a.co/gxwT3ep
Port tube: http://a.co/45KbwTQ
Binding Post: http://a.co/6GrDdZF
Kicker Speakers: http://a.co/7RHq1mJ
Bluetooth 30W Amp: http://a.co/7VBiCXg
2.1mm Power Plug: http://a.co/3lmjQ9c

The wiring was pretty simple.  I located the brown radio + lead behind the dash and connected that via bullet connector to an inline fuse and then to the toggle switch and then to the 2.1mm plug that goes into the 30w amplifier.  The negative side is simply connected to vehicle ground.  This wiring scheme makes the toggle switch work no matter the position of the ignition key.  If you wish to connect the toggle switch to a switched (via ignition) circuit you can run a lead off of the top of the fuse box on the inner fender well inside the engine (pictured here).

Top set of connections = ignition switched. Bottom = constant-on via battery.

Here are some additional pictures that might help with sizing.