Use glue to attach the cover and then pop some rivets around the perimeter especially near the seams. Afterwards you can clean up the bottom of the seat pan by gluing a piece of black trunk liner on the underside of the pan.

Hello John, You could first use some 'yellow glue' to attach/fasten the cover to the metal pan. Then to have it more secure you could drill little holes to put in some little screws (7mm /5/16" long approx.)
More ideas on a Google link: https://www.google.com.mx/search?q=metal+pan+for+a+motorcycle+seat&tbm=isch&source=iu&ictx=1&fir=-XxOIS3WeAvjgM%253A%252CPBllvJ2rbH2pcM%252C_&usg=__spuyfk2_TduAFJuirdXduHsQplI%3D&sa=X&ved=0ahUKEwiR7bf74snYAhXMSyYKHTpqD9gQ9QEINDAA#imgrc=-XxOIS3WeAvjgM:

Ok thanks to both of you. That’s kinda what I was thinking although I hadn’t thought about the rivets. I like that idea nice and clean. So on the rivets, I’d clue the bottom of the cover down, then drill the holes thru cover and pan at the same time and then just put a normal 1/8 rivet in there?

Probably best to pre drill the holes, then glue cover down. I'd use 1/8" large flange.

I glue the cover and pop some rivets on it. I use rivets with bigger heads than normal about 1/2".