I designed this storage box so that it fits on one piece of 30cm x 30cm of plywood.
I used 3mm beech plywood here as it tends not to warp as much as others.
Simply place in your machine and mark the area.
You will need almost all of the plywood sheet.

I decided on the lightning bolt as an image but you can choose whatever you want.
I specifically wanted the images not to be dead centre as I wanted a little variety.
For this template, I have re-centered the images though.

This is where you need to be a little careful.
At first glance the sides may look the same size but they are not so you need to ensure that you fit them correctly. They do fit (promise!) as you can see from the images.