Can't Put a Base on Topography model

Hi all,
I’m a beginner and I am having immense trouble creating a base for this topography model I have made. I have boolean unioned all the layers together, and have tried several methods for attaching a base - made curves for each side and lofted them (didn’t work) - made a solid cube around the model and tried to boolean difference (did not work). Also, is there a way to fix the non-manifold edges? Any help would be very appreciated. Topo Model.3dm (1.6 MB)

Hi -
The object in your file is a bad object. You can run ExtractBadSrf on it and at least get rid of those.
Apart from that, I’m not sure how you managed to create the object. In essence, your model consists of boxes that are stacked on top of each other but only sharing one edge with the box below. This leads to the non-manifold issue. You only need the “ground” surface and the edge surface connecting two levels. You can delete the rest.

Then you should be able to use the remaining surfaces to trim a box. Use the Intersect command to check where there are gaps that need fixing.

In the attached, I’ve used BoundingBox to create a box and scaled that inward a bit to get 4 simple vertical walls for the base.
Topo Model-maybe.3dm (1.7 MB)