I had this happening a couple of years ago. It was really annoying to find the mess of cracked eggs in the nest box and have to clean everything out.
I purchased some "standard nesting pads" from QC Supply. I put a nest pad in the bottom of the nest box, add about an inch of pine shavings, and then add a bowl-shaped layer of hay. Even when the hens scrape down through the hay and pine shavings, there will still be the soft landing area of the nest pad when they lay an egg. I haven't had any problems with broken eggs in the two years I've used these. Well worth the investment!