I ran across these Japanese Nest Egg Gourds that were used as nest eggs to encourage egg laying that were popular in the 1880s. They are white and about the size of a hen's egg. http://rareseeds.com/vegetables-d-o/gourds/gourds/japanese-nest-egg-gourd-13494.html
That link in the post above is not working. Here's a blog post on egg gourds and how we used one to encourage our hens. http://www.avianaquamiser.com/posts/Gourd_nest_eggs/ Of course they dry out after a while....maybe one could inject some heavy weight epoxy to keep it feeling heavy, although it would cost more that a golf ball at that point.