Ideas on how my object became hollow I started out with a simple cube and edited the vertices to make it look like so...



I think somewhere along the way I did something to make the object hollow, because when I cut an object out of it (Boolean Modifier -> Difference), you can see the insides (as follows)...



Is there a way to add the "fill" back to it so that it's solid?

Blender Source File
I hate answering my own question, but in hopes it might help someone else...


Create a giant cube that surrounds all edges of your problem hollow object.
Add a Boolean + Intersect modifier to the cube with the target object as your hollow object.
Apply the modifier and delete the old hollow object, and now
you should have a filled object on the new "cube".