I always use a crisp box from lidl, (i work there so there are plenty to choose from) they already have a flap cut in them and lids if you want one. I line it with plenty of newspaper and a piece of vetbed. My cat always births in the night so when shes had them i let her settle untill morning then i use a fresh box an piece of vetbed . i change it weekly then.
Good luck with your next litter