Polypanels are great for building anything you want. However, the larger the object, the more polypanels are required.
This plank connector is a simple addition to the already existing square polypanel to build larger objects. Even more things are possible in combination with the rod connector that I designed: https://www.myminifactory.com/object/3d-print-92680
You can build small tables, side tables, or a storage box for instance.
The uploaded STL file is designed for planks with a thickness of 12 mm and has a tolerance of 0.2mm for a nice fit. The structure can be made more rigid by adding glue to the connector and the plank.
No support is required while printing.