So I have created a widget for use with ThingWorx and my structure in my zip file is
Top Level Folder named - ui and the metadata.xml file
inside ui I have a folder with my widgetname (for instance textboxupper in my case)
inside ui, I then have
images folder and 5 files
I think you are missing the ui folder level, just a thought.
If I were you I would start basic by just creating the exact same control as another control and renaming it and then doing the import and validating that everything works. I know some of the developers here have had problems when they have forgotten to rename an element, you might want to validate that everything is renamed properly as well, just a thought.