Aspen. Kings love to burrow and they poop ALOT. Paper towels or newspaper, you'll be changing that out every day. Not fun.
Aspen is a great, safe, and absorbent option. You only have to spot clean the poop (if you an find it!) and do a full clean once a month or so. Just watch them when they're eating and make sure they're not ingesting aspen. Use tongs to pry it out of their mouths if necessary. Aspen FTW!