Hi Derek, yup the problem is exactly as you suspected - the shape you are using is not a completely sealed solid, it is only an open surface.

The quick way to diagnose this is to just look at the object type indicator here:



That's one of the new features for MoI v2.

That type indicator will show "Solid" when you have a solid selected - in your case it says "Joined srf" to indicate that you have several surfaces joined together but not totally sealed everywhere into a complete solid.


Then to get more specific information you can set up a keyboard shortcut with the script on it from here:
http://kyticka.webzdarma.cz/3d/moi/#SelectNaked

which will highlight "naked" edges - those are edges that are are not joined to another edge, making an opening in your object.

In your object there are quite a few of them:



How was your object created? There seems to be some pieces missing, like the side walls of the hole on the side are just not there.

- Michael