It wasn't too pretty, but I have bolted them together. I took a piece of thin metal (I think it was HVAC trunk line tin) and bent it to follow the curve. Then I drilled out holes on either side about every 4 inches and bolted it tight. It held until the old man blew the engine. You could rivet it as well.
Why couldn't you just get some fiberglass mat and fiberglass the back of it? One of the neighbors has a combine that has 80 percent of it held together that way.