Hi @marscho
the spatial mapping module is designed to work in bigger areas, 0.8x0.8x0.8 m is a very little workspace.
I suggest you write your own point cloud fusion algorithm exploiting the iterative closest point (ICP) algorithm that is well suited for this kind of configuration.
The ICP algorithm is available in the PCL library, in the Open3D library, OpenCV, and in many different open source implementations on Github.