i need help with a hole!!!

i have a 97 sonoma stepside. it has a large hole on the fender well ( drivers rear panel. i was thinking about using bondo but the hole is big, i was thinking about using a piece of sheet metal, i have a welder. what size sheet metal should i get (thickness) and should i tack weld the hole thing then grind them smooth then apply the bondo, i just need help because its on a curved area. any help and writeups on this would be great thanks