Do you have a sedan or a coupe? I have a sedan and 30" is the limit on the width because of those little ledges that stick out towards the back of the trunk. You could build a box to span the whole width, but your box would need to have cut outs in the back for those ledges so that it can sit flat on the trunk floor.
Unless your planning on building a huge box, the possiblities are endless. I've seen some nice walls built around the face plate to make it sit flush with the trunk....combine that with a nice false floor for the amps and you have yourself a perfect setup worthy of vinyl or paint.