How to make a "Positive" mold of a hollow mesh?

I’m trying to make a “positive” version of the bunny. The .stl is a single mesh but has the bottom truncated. I’m trying to “fill” in the void and remove the original mesh.

I’m not sure how to fill this in. I was going down the road of finding the face normals but not sure how to isolate the inner faces. Any guidance would be appreciated.

cutbunny.stl (1.2 MB)

Is the cut on the bottom of the bumny planar? (I’m not at my pc atm)
If it is… Get naked edges of the mesh. Create a Boundary Surface. Brep to Mesh. Join and weld to the original mesh

If that’s what you are trying to do? Basically cap the hole on the bottom?

Do you want the outside mesh only like this? cutbunnyouterDC.3dm (246.5 KB)

Explode the mesh
Select the inner mesh by clicking on it and deleting
Select the bottom mesh by clickingon it and deleting
Join the remaining meshes

Solid mesh with bottom closed cutbunnysolidDC.3dm (267.3 KB)

Start with outer mesh from above.

Above were done in a couple of minutes using Rhino commands.