Customizing in a creative and cohesive way takes time and it improves with practice. But I have a tip called the Offset technique that can help you design faster some beatiful effects.
It consists of creating two offset lines on the inside of the customizable skins without bleeds, in every single shape of your project

Doing that, you’ll have three different polygons (one inside the other) for each side of your box. With these three polygons you can create different customization styles by basically adjusting four parameters:
- Fill

- Line color

- Line thickeness

- Line style

By alternating and combining different styles, you can improve the design of your project and get creative results in a easy way.