I'm trying to visualize the result. I hope you will bear with me here, and not think I am being overly critical of your suggestion :-).
I'm trying to figure out what exactly it is that worries me about the current situation, and what effect the angle iron solution would have on it.
I'm concerned that the weight of things I put in the loft right now is pulling the entire loft downward, and especially the front center. I worry that by pulling it down, I am pulling the sidewalls inward and putting stress on the seam between the raised roof and the side walls.
I don't know whether this is something I really need to worry about. The raised roof is held on to the side walls by screws, about every 3 inches. So I guess downward pressure on the center of the loft is resisted by all those screws along both sides of the van.
If it is a problem, how would the angle iron sandwich change the situation?
I think it would transfer all the stress to the two section of the loft edge, one at each end of the angle iron. The loft metal would want to tear at those points.
I don't think it would change the stress the loft contents put on the side walls. I think that would stay approximately the same.
Also the weight of the angle iron and the strap bouncing around up there as the van goes down rough roads might not help matters.
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Maybe one of you will tell me that I'm worried about the wrong thing, and that the angle iron project is a solution to a different problem, one that I am not aware of.
I'm used to thinking about houses that are made out of wood and don't go down the road. I also feel like I don't really even understand what holds the box of this van together. It seems like it is about tension rather than obvious post and beam weight bearing. Either that or nothing is holding it together and it's just as flimsy as it sometimes feels...