Panels of the invention are used to form large capacity enclosure sidewalls. Trailers, trucks and cargo vans are examples of such enclosures. Intermediate framing support is not needed. Each panel has a polymeric core with a first metal cladding and a second metal cladding. The core has opposed longitudinal configured ends, each of which has a stepped shoulder which extends the length of the panel. The metal claddings follow the contour of the stepped shoulder. The stepped shoulders are offset from the first face of the core approximately one-half the thickness of the panel. In assembling a walled enclosure sidewall, the panels are positioned so that adjacent stepped shoulders mate together to create a flush face surface. Joining means are used in the stepped shoulder areas to permanently join together adjacent panels. The joint formed by the attachment has a total of four metal cladding layers and two interposed polymeric cores to give the joint sufficient rigidity for the sidewall to withstand normal forces during use.