Customs bonding dissimilar materials

So I have a roof that the front is steel, but the rear is aluminum. I can use the existing wood roof rails to tack the two pieces in place permanently, but am looking for suggestions on covering the seam this would create. Fiberglass? Bondo? Some sort of Targa strap thing?
Concerned about the seam showing up later, thru the black paint.

 

SEM general purpose panel bond works great in those situations