I'd recommend using a nest pad in the nest box. I put these in the bottom of the nest boxes, then add pine shavings and hay. Even when they scratch down to the bottom and remove the pine shavings and hay, there's still the nest pad for the egg to land on as the nest pad stays in place. This has greatly reduced the number of broken eggs in my nest boxes.
These pads are very easy to clean. I just bang them against a tree to remove the pine shavings and hay, then hose them off with water.
https://www.qcsupply.com/420355-standard-nesting-pad.html