Converting .xyz file of surface points into stl body

this is my first time asking a question here, I would be really glad if someone out there could help me.

What i have are evenly spaced coordinates of a rough surface, that I want to use in an Openfoam simulation. My aim is to create a .stl file that I can mesh with snappyHexMesh, but I am also open for other suggestions (maybe a Salome solution?). I converted the coordinates into a .xyz file and I am now trying to create the stl file with MeshLab. Here I ran into multiple problems.

First, I just have the surface coordinates but in the end I want to have a body (like a block with the desired surface). Second, if I use 'compute normals for point set' and then 'surface reconstruction: screened poisson' a square surface is created where the desired surface only exists where the square and the point data intersects and a smooth surface is added at the sides, which is not what I want. I can't post a picture yet, but with both the point data and the poisson mesh visible it looks like this:

         ___________________
        |^^^^^^^^^^^^^^^^^^^|-----rough surface as point data
        |^^^^^^^^^^^^^^^^^^^|
  ______|^^^^^^^^^^^^^^^^^^^|______
 |       ^^^^^^^^^^^^^^^^^^^       |------reconstructed surface
 |       ^^^^^^^^^^^^^^^^^^^       |
 |       ^^^^^^^^^^^^^^^^^^^       |
 |       ^^^^^^^^^^^^^^^^^^^       |
 |       ^^^^^^^^^^^^^^^^^^^       |
 |       ^^^^^^^^^^^^^^^^^^^       |
 |       ^^^^^^^^^^^^^^^^^^^       |
 |       ^^^^^^^^^^^^^^^^^^^       |
 |       ^^^^^^^^^^^^^^^^^^^       |
 |______ ^^^^^^^^^^^^^^^^^^^ ______|
        |^^^^^^^^^^^^^^^^^^^|
        |^^^^^^^^^^^^^^^^^^^|
        |^^^^^^^^^^^^^^^^^^^|
        _____________________

I am completely unfamiliar with MeshLab and in all the tutorials I found Screened Poisson just did what it was supposed to do.

Thank you very much!