Now, obviously the most logical answer is internal geometry, but this model has none.
The larger hole is for a power bank, and the smaller for the cord.
I take great care in my models so when something goes wrong, it's quite upsetting. Especially after having spent so long making it.
Hell I even tried netfabb. That didn't work, either. I'll include that file as well. I don't think netfabb actually changed anything.
I just don't understand it. There shouldn't be any reason for that hole to get filled up. In blender, that hole was made by extruding a face downward and