3D print a terrain surface : From a 2D surface terrain to a 3D printable object with a flat base

Question

I am working on a project where I take lidar data and I want to 3D print them. For that I need to create a mesh from a 3D point cloud and add a base to that surface. Now, I already have a python code that transform the cloud point into an acceptable surface mesh (there are still imperfection but that is okay).

But now I need to add a flat base to that surface mesh so I can actually 3D print it, but I did not find any program or code that do that. I tried to solve it myself in the following way :

1. I do the 2D convex hull of the terrain
2. I take the points of the convex hull and create the base surface bellow the terrain
3. I create the 'walls' that link the flat base to the terrain

But because there is some relief between the points of the convex hull of the terrain, I cannot link properly the base to the terrain.

So, do you know a method that can extend the 2D terrain mesh to make a flat base ? (A watertight mesh would be nice). (Also I would like to manage that in a python program)

P.S.

I tried the 'Extrude base' from the MeshInspector program but the resolution greatly decreased so it's not really suitable.