I’m working in the same area on my car. Just removed the front half of the fiber board and 10 pounds of glue that held it on. I’d suggest that the channel is there to stiffen the roof panel and keep it from flapping in the wind. I’d further guess the channel, fiberboard and roof panel are an engineered solution that all work together. The fiberboard is sandwiched in between the roof panel and the supporting channel.
How are you planning to attach the aluminum sheeting to the roof panel?