OK, this is not a detailed lesson..........it's only describing a technique.
I used the two images to visualize what I wanted. I placed the box image over the truck image and lowered the opacity.
I then added some guide lines to show where I wanted the curve to extend.
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On a new layer, I used the Pen Tool to draw out what I wanted. I used the path setting here but I should have used the shape setting.
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Once I had the path were I wanted it, I filled the path with the color of the original truck.
This is the new truck layer combined with the flowing box layer.
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I'm not certain of exactly how you wanted the bottom of the truck curved, but this give you an idea of one way to accomplish the task.
Not sure if this helps. Please feel free to ask any questions.
