Have you guys considered talking with Pro installers? I ask a a good installer can make a system dissapear inside a car.
Several materials are acoustically invisable, others trap the highest frequencys and for this application work perfectly. I don't believe the loss of the triangles resonance will matter much with the engine so close to your ears.
A remotely controlable head unit eliminates the need to see the headunit, so it can be stashed behind the dash, or in a kick panel. A small CD changer can make life simple, as they can be easily hidden in the Console.
Speakers, you have several options, hidden in the dash under the acousically invisible fabric firing at the windshild and reflected to you will provide the sound staging most of us are accostomed to. If you don't care where the music is coming from 2 small 3.5" or 4" full range speakers mounted in the head rest. With some from of bass over-ride to prevent major distortion they sound moderatly good.
Bass Blockers are a passive form. They strip the low frequencys and simply plug inline at the speakers connection.
If you must have the low frequency sound a small 6" driver could Possibly be fitted in to the back of a seat. I say possibly as I'm unsure how much room there is behind a seat in a 40, if any. another possible option is stashed under the dash or even in the pass foot well.
Jim