I have a panther that has been gravid for a couple months now. She is eating less, but still eating, and her belly is big. I don't see marbles, and right now she's in a bucket.
The idea is to leave her in her cage with a proper lay bin and if she starts digging in it seriously (is it's the hole she has selected) then move her to the larger lay bin...although most of them will lay the eggs in the one in the cage if it's set up right. The one in the cage should be opaque and at least 12" deep x 12 "x 8" filled with moist washed playsand. The moisture level should just be enough to be able to hole a tunnel. Don't let her see you when she digging....it can cause her to abandon the hole.