Else discover the speedy Blender (free) Method! (with some hairs losted...this prog is a mess) :)

Import OBJ (no "Marching cubes" ) from MagicaVoxel : Select the Object ..Edit Mode / Face .... Press F
.
that all : triangulation is gone ... you can export ! :)

_importOBJ by Max Smirnov for MOI
You have a solid (no subdivisable) perfect for nurbs operations!



Some moments of inspiration later! :)

PS Find the ultima Blender Method : a nightmare for find the good process!
- merge by distance - unlimited disolve - clear custom split normal data!

Woks fine directly inside Blender for OBJ or PLY (Marching cubes) in one pass!

Thx to LOLock
https://i.imgur.com/LceY8nu.mp4

So in MagicaVoxel you make this volume in 1 second!
And can of course modify it in Moi3D (if wanted) by import an OBJ without triangulated faces!
_importOBJ by Max Smirnov