I would put some straw or hay in there.
The girls will be spending a lot of time in those boxes (hopefully!) and they will want to be comfortable and cosy.
MY BO can take up to an hour and a half before she has settled in, laid her egg, sat on it for a bit in a trance, and then moved off.
My RIR's are a lot quicker, but none-the-less they still like to flick bits of hay around and bury down nice and deep into the hay to lay their eggs.
Krista