Print out the large box with the open end up. Print the long triangular piece, and glue it to the interior edge of the large box. Print out the wall mount.

You may move the triangular piece further into the large box to make the shelf hang closer to the wall.

This box uses french cleats. https://www.bobvila.com/articles/french-cleat-wall/

Do not exceed five pounds or 2.2 kilograms.

Space in the center fits Plastic Bubble Spirit Level Vials, 8x35mm that are available on Amazon.