I am testing the plugin and unfortunately, when I set it to use the "thumbnails" template provided, the plugin is retrieving the full size feature image instead of their square thumbnail.
But I realised the PHP Custom thumbnail template is using those square thumbnails. Unfortunately, it's design isn't as good as your template.
I had a look at your code and couldn't find out why it's retrieving the full size image and then resizing it to 120*120 instead of using the square 150*150 thumbnail and then resize it to 120*120.
Any idea ?