Easiest is to use the front of the stage. Build a frame halfway around the "pit" but leave the downstage half open, so the audience can still see. You can build fully around the bottom so you can obscure whatever cushions you'll have for the actors to land on.

If you have the tech, you could do the above, but cover the front with a scrim. Then light the actors in the pit from inside the pit and the scrim will become translucent, but only when you need it to be.

If the stage front or audience seating won't allow you to build a pit from the edge of the stage to the floor, you could conceal the pit in another large set piece. I'm not familiar with that specific script, but surely there's a place for a large log, a hill, a honey tree, or something that can act as host for a large pit? You can either have the set piece spin to reveal the pit, or use the scrim method as above.