Regarding your question about the material choice for the underbody panels (aluminum or plastic), how about both?
Given the choice I'd prefer metal for durability, but at the same time I know the plastic is likely to cost less and would do better with any heat that might be generated by boxes, and durability would only be an issue if one is running over things consistently or having to remove the panels often for maintenance. But if you can do both, I say do both and that way we have an option for preference. Maybe see if there are some existing points you can use to bolt these into place, or in the case of plastic, maybe look into some sort of snap-fitting style?