Another option would be to create a large morph mass for the site and then use the mesh you made as an SEO cutting plane.
Step 1. Create a large morph box that extends the full width, length and height of your mesh.
Step 2. Change the settings of your mesh to be just the mesh plane...don't choose the option that gives it thickness / solidity below. in other words it should just be a face, not a solid.
Step 3. Place the mesh on a hidden layer that is not visible in your regular presentation views.
Step 4. Create an SEO with the morph as the target and the mesh set as the operator, with settings set to "subtraction with upward extrusion'. Then execute the SEO.
Because SEO's remain 'live' you can modify the mesh any time you want and the morph will automatically update.
Jeff G
Archicad 27 USA (full), Macbook Pro (16-inch 2023, M3 MAX, 128 GB RAM)