When I was little, my parents found a salt shaker that was vaguely egg shaped. The hens sat on it no problems. We learned that plastic Easter eggs, even filled with sand, evetually became brittle and broke.
My brother got a handful of golfballs and he's been using those in his hen nests.
Last year, knowing I'd have laying hens, I looked for eggs at Easter time at drug stores and the Dollar Tree. I was using two glass eggs and a stone egg until my broody hen decided to try to hatch them. I brought the three fakes in the house and Ms. Broody is now hopping off the nests. I hope she'll start laying again soon.