Add images, text etc to to the sides of the box. Be careful not to get to close to the edges since you still want the box to have strength. Measure thickness of the board you are using for the handle and change the the width of the rounded rectangles to match. Make sure when you make this modification doesn’t change the center of the holes. If you aren’t going to round over the handles or the box then remove the rounded corners.
I suggest painting prior to assembly of the box for ease. After you finish painting glue the box up. With the box together without a handle, use a round overbit on a router on all corners include the handle. This gives a much smoother feel to hold onto. Now glue up the handle and apply final stain/poly.