TheOneBag Empty Inventory Slots

As part of a learning attempt, I've been working on modifying TheOneBag to my personal tastes. One of the features of the normal inventory bags that I'd like in TOB is the empty space art asset. Any idea where to find this, or some quick and dirty hack to get said space in place?

Currently, I've got the background of the empty spaces as a grey color - but that's really not ideal.....