How to create a flat base to a photogrammetry model in Blender? I am trying to create something like this in Blender, but I don't know how exactly.

I captured a model of a WWI bunker with photogrammetry and processed it with Agisoft Metashape but I like to add an “earth base” for a nicer presentation. I imported the OBJ file into Blender but I'm having trouble making it work (I'm kind of new to Blender).
Extruding the model downwards gave bad results, so I tried duplicating the model, making it flat, selecting the edges of both the model and the base with Edge Loops and then Bridging the Edge loops but that gave really funky results (I think not all edges were correctly selected, resulting in an uneven amount of edges between top and bottom).

Could anyone propose a workflow to achieve this in Blender?
What I do usually is profit from the new vertex group options for generated geometry in the solidify modifier from version 2.83+.
First let's say we have an ocean modifier on an object.

Create a new vertex group.

Add a solidify modifier, set the Shell Vertex Group to your new group, and the Thickness to 0. This will effectively duplicate your geometry in place.

Now add a plane at the altitude at which you want the bottom of your foundation to be. Scale it so it is larger than your terrain.

Add a shrinkwrap, set the target to your plane, vertex group to your new vertex group, set the mode to project on the Negative Z axis. Hide your plane.

Now add a new material for the dirt/soil, assuming the first one is your photogrammetry raster info.

Get back to the solidify modifier and set the material offset to 1. Result :

With &quot;proper&quot; shaders:

Note : The geometry will be ugly if viewed from upside-down but what you can do is check &quot;Only rim&quot; in the solifidy modifier to only keep the rim of the object.