For much faster modelling/designing than you can mange in SM directly, you can now create your models in MagicaVoxel and use this program to convert the resulting .vox file into a .smtpl file to import into SM.
- Game version:
A side benefit is that with MV you can model on very low powered machines, too.
The zip also contains a custom palette for MV, and a custom MV GUI config that shows block types in-program.
There's a thread in Mods for any discussion: Vox2Smtpl: convert MagicaVoxel .vox to .smtpl