I used door skins for the dome gores.  I first made a template of heavy paper for each gore.  I simply taped the paper to the ribs and marked it from the inside of the dome.  I added 1/2 the thickness of the ribs to the outline and cut the template out. I use this to cut the gore from the door skin.  I used construction glue and 1 inch deck screws to attach the gores to the ribs and dome ring.