Leaves, long, grasses, hay, etc. can all be used for nesting. Many hogs prefer to make their nests themselves, so maybe just put a little in to start with to give them the idea and leave lots of nesting material nearby which they can take in themselves.
Hope someone moves in soon, but don’t be too disappointed if they don’t move in immediately. Some hogs will move in quite quickly, but others like to wait until the hog house has become part of the scenery.