I used a mid 80's GM full-size station wagon roof (mine was an Olds but the Chevy, Olds, Pontiac were all the same.) These roofs have ribs ....maybe you want the insert plain..??) The ribs gave the roof a little more strength. These roofs might be getting hard to find ???
I used a sheet of I believe 18 gage steel,,,,trimmed the top bows down to remove some of the arch and used a welding bottle to get a curve in it..I left the tack strip channel under the top so if someone ever want to remove the steel fill-in panel they can...