How can I allow building on top of a custom cube in UEFN?

Josh Grant just answered a similar issue here

For quick reference:

From my experience, this is the best flow for making custom meshes that support player builds:

  1. Create a Blueprint and select the Building Prop option.
  2. In the Blueprint editor, select your mesh
  3. Enable ‘Is Player Buildable’
  4. Enable ‘Do Not Block Building’
  5. Enable ‘Structurally Support Overlapping Actors’
  6. Compile your Blueprint
  7. Drag your Blueprint in
  8. Build on it!