[herr_oberst]

Do you even need to create a crush zone in the tube? Won't the tubes be the path of least resistance by their very aluminum nature? (Assuming you will use all of the original mounting points with similar brackets)

How about a slot (about a quarter inch wide) running the length of the tube at the bottom to create an area for the tube to expand in case of impact? Or 4 horizontal slots.

I always thought an elegant solution to the bumper shock dilemma would be many pieces of thin solid aluminum rod welded together in multiple triangles. Think of stage light rigging, but on a much smaller scale. Light, strong, capable of withstanding stresses in many planes, and could also be engineered with a crush zone to save the tub. Probably too labor intensive to be practical.