How about a custom built 12' high x 36' long smooth wall box w/ I Beam  spreaders...

 This is a Titan 12" smooth wall box that has been pre-stressed during construction for an outward-bow to resist the load over the long length. At full load, each panel is laser straight, not bowed-in like an inferior stitch welded box would be.     Nothing works like continous welds... think about it.

Notice the placement of the bottom spreaders; they sit right on the trailer bed, providing perfect clearance top & bottom during transport.   Who else could dream this one up?

                                                No one but Kundel...

The Titan 5 Lite is way to go if you are typically only going 8 to 12 feet deep. This is a great light-weight box for the 40k to 50k size excavators.

 
 Titan Brute series is the box for the serious pipe contractor. Extra heavy-duty through-out. Standard sizes: 8x16 to 10x36. Any size custom.