I use rocks
Seriously, I have a couple roughly egg-sized-and-shaped stones I picked up in the horse paddock. Color and shape are not quite right, and they are on the small size, but they work great, seemingly better than the plastic easter eggs I've tried. (When I put an easter egg in one nestbox and a rock in the other, eggs are nearly always in the rock's box
)
Pat, "Born Cheap"
Pat, "Born Cheap"