Hi there LampRabbit,
Yeah, the game I'm working on is a point-and-click adventure so there will only ever be one of each item and they'll all be listed before hand. You could limit the number of slots pretty easily by comparing an Array List Count with a predefined limit perhaps?
In terms of layout my approach is definitely very basic, I'm just having a horizontal line of items to scrub through. You could maybe have a separate array of Vector3 values to call on? Or even just add a few extra steps to the incrementation method I used, like, after every 4 slots reset the X and adjust the Y down to give you a new row?
Glad you found some of it useful though!