the alpine amp I mentioned has a height of 2.5" - a box could easily be fabbed with a lip on the top that would totally conceal the amp within its shroud. The footprint of hte box would be 10x12 or so - slightly under a cubic ft of airspace will sound nicely. If you were to face the sub forward in the trunk you would have two visible sides of carpet and nothing else. The wiring and amp would be hidden behind the face of the wood and the wires along the side of the trunk neatly bound.

The problem is not the infinity components, its the lack of airspace that restricts the truly low frequency output of the basslink. if you listen to punchy rock you can probably get away with it, but throw in some LP or some rap and your =) will turn to =(