Align two Uprights at 90° to each other (figure 1) and fasten with a couple of wood screws 15mm from the outer edge.

Tip: If the Upright is deep enough (to allow for a screwdriver), you can hide the screw by attaching it from the inside (figure 2). This must be done prior to inserting an Infill.

Note: You can join Uprights of different depths in the corner to make an arrangement more interesting.