Moonspear Fire may save you

Ness drew a rough diagram in the soil and Wildfire peered at it speculatively. She was no engineer, though she didn't understand how they would make the bottom of the pit wider than the so-called bottleneck. If the earth was wet, it might be possible to pack it, otherwise it seemed to her like it would just collapse in on itself.

Before she could comment on this, Ness began babbling about cougars. Wildfire's lips twitched in amusement. "Charon would like it if we came up with one big enough for bears," she said, then added, "but I don't see how that's possible. It would have to be enormous to effectively trap a larger predator."

Glancing back at the image scrawled in the dirt, she asked, "I think this is a great idea in theory but how do we go about this? How do we stop it from collapsing in on itself?"