Conex come in different lengths and heights.
They come in different steel alloys and aluminum.
I could see welding brackets to your can, with matching mounts on the truck frame.
I imagine you want to weld on different doors. Windows. A porch.
A simpler approach might be to acquire a 'straight truck' with a van box or reefer engineered and factory-mounted on a MDT. A P&D truck designed for pickup-and-delivery probably comes with your lift-gate.
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We live near the outskirts of Eugene, Oregon. One of my hobbies is walking through our local wrecking yards aka 'vehicle dismantlers'. My favorite is the MDT and HDT dismantler Anderson Brothers. They have boxes such as I describe. Unfortunately, after the engine quits or the front-end gets smacked, the box has zero value, so they get crushed and scrapped.
I hope you keep this thread active so we can see your progress!