Great to see a few package being useful. I never know if anyone uses these things I throw out on github but great to see vertices-to-h5m and dagmc-file-inspector being useful.
I think Jiankai has solved it but I just wanted to add that there is a auto bounding option as well.
Traditionally DAGMC geometry was made with a hollow cube surrounding the geometry which had the material tag graveyard assigned to it. This acted as the vacuum boundary and killed particles when they got to this outer volume. This functionality is still supported so you could make another set of triangles to encompass your geometry.
More recently openmc was able to contain a graveyard free dagmc geometry in a cage surface like Jiankai example above. This makes visualization easier as there is no large encompassing dagmc volume.
Really recently we added this pull request which simplifies things further
This allows combined universe to be made in one line