Apparatus and methods for joining panels are presented. Panels may be commercial off the shelf (COTS) panels and/or additively manufactured panels; and panels can be joined using an additively manufactured interconnect unit. Notches and protrusions can be formed in the panels and in the interconnect unit to facilitate the connection of the panels with the interconnect unit. In another approach a ribbed triangular prism can be manufactured using three dimensional (3D) printing. The panels can then be joined by inserting the ribbed triangular prism between the panels and by using an adhesive for bonding.