Hey guys, I used the search because I do not want to open a new topic.

I made a small tutorial that describes how to export marching cubes models from MagicaVoxel to Moi3D for Rapid Prototyping in Moi3D -> http://www.8bit-labs.com/article_t/3/
The interesting point in that story is that I use a dirty (not optimized) polygon mesh natively to generate a waterproof solid in Moi3D by doing 2 Boolean operations.
You can read the details in my tutorial.
Here is an image of the process in Moi3D and a short version of the description (Moi3D part) ...

1 Import obj file in Moi3D [4], select all faces and join them via join command.
NOTE: in the original tutorial I use a step file from Solid Edge. I generated that step file based on an obj file that I imported to Solid Edge before to export it as step file because Moi3D can import them out of the box. This step file is dump, means all faces are polygons itself. You can see it in [4]. Frenchy Pilou already gave me session on Discord that I can import obj file by a script of Max to Moi3D ;) .
2 Construct a solid box around the whole imported step/obj model.
3 Select the solid box, hit Boolean operation ‘subtract’ and use your imported model as ‘objects to subtract’. The result is a solid box that encapsulates empty volume corresponding to the imported model [5].
4 Construct a 2nd solid box, select this one, hit Boolean operation ‘subtract’ and use your result from the 1st Boolean operation as ‘objects to subtract’. The result is a solid model [6] of your original export from MagicaVoxel.

I wanted to share this small work around because I was really surprised that it is such easy to transfer a polygon mesh to a solid body in Moi3d; and such robust.
Thanks for your time and have fun!