1-Take the final mesh(before thickening).
2- explode the mesh.
3- take the faces.
4- compute the centroid of faces.
5- check if the centroids of the faces are inside your boundary(you have to extrude your boundary to make a volume out of it).
6- cull the faces that their centroids are inside the boundary.
7- join the rest of the faces.
Thanks for your help it took me a bit to get the steps as a beginner but i cant tell how helpful that was. i have another question about the last step cause i didnt get it. What do I have to join with what? Sorry for that dumb question its 2 am.